1. I bought the U-32000, which says on the box and on the Owner's Manual that it holds 2.5 gallons. It only holds around 1.75 gallons, which is a BIG difference. Down in the body of the item page they now write that it does hold only the 1.5 gallons, but up under the main item description where it REALLY counts they still claim it is a 2.5 gallon tank (same model number). I believe this would qualify as an Inaccurate Web Description. 2. I paid only 1/2 the price for mine 13 months ago, but only just started to use it a month ago, which means the warranty is over. I thought costs were supposed to go down on such things, especially since they didn't change the main page claim about how much it holds!
3. Unless you religiously clean that absorption sleeve often, really often, you will have a mess very quickly and will likely need to buy them constantly. They DO now sell them here on Amazon: Optimus U-30002, at 6 for $16.99. That will really add up and you MUST use a sleeve. I've decided that's an unacceptable price, particularly with these other issues. I'm only sorry I didn't do enough homework before I bought this. 3. It creates a fine dust that gets on everything. I do have hard water and I do filter it, but no dice. Unless you own your own machine to distill water, you'll pay for 2 gallons, twice a day if you run it on high. Can you imagine the price of that! Maybe soft water will work; I have no way of knowing. I own a cool mist Holmes that I love. It's cinch to clean, needs no sleeves or filters and lasts for 24 hrs (1.5 gal) on high. Same price. I think I'll buy another one of those.

I just ordered another one of these humidifiers. I really like it; it is efficient, quiet, and easy to use.
The only reason I gave it four stars rather than five is that after three years the float switch failed. The switch shuts the humidifier off when the reservoir empties. The switch on mine failed safe, meaning that it shut the humidifier off when the water ran out, but now it won't switch on. It can't be economically repaired, so I ordered a new humidifier.
Our water is moderately hard, so in a week the humidifier wick accumulates mineral deposits. Dealing with this is easy: Remove the crusty wick, and install a clean one. Let the mineralized wick dry, put it in a cup of vinegar for a day, rinse it, dry it, and it's ready to go again.

Love these "“ they are very quiet, dependable, and well designed. This Optimus U-32000 model seems to be a direct replacement for the now obsolete Kenmore #15123 humidifiers, of which I have had two in operation for the last 8 or so years. Both worked flawlessly over that period, though with light duty "“ ie only used during cold nights of during periods of low humidity. When my Kenmore quit I looked at all the new humidifiers and reviews and could not find any that were quite, had a humidity control, and were easy to fill. I tried a cheap one and was very dismayed by the loud and random (especially at the "low" setting) "put..put.put"¦.put" noise it made when boiling the water, by the crazy fill tank shape that made it impossible to lie flat when adding water (thus you had to hold it up which gets heavier the further you fill it), and the large fill cap that was very difficult to turn due to the tight seal it required. So I was ecstatic when I discovered the Optimus on line.
Optimus U-32000 Likes:
"¢ Very quiet "“ only a soft "shshshshshsh" sound with a very occasional soft gurgle. Perfect for sleeping.
"¢ Dual handles on the water tank "“ one on top and one on the bottom make it easy to carry and refill. And unlike many other tanks, this tank lies flat while filling it. The U-32000 has updated the tank design from the Kenmore #15123. The Kenmore version had a slightly tapered tank which caused a few reviewers to note that failure to place the tank squarely on the base caused it to not fully engage the tank-in-place sensor. I liked the tinted blue tapered Kenmore version better for appearance, but the newer tank design works better and also has a clear base which makes it easier to see the water level while refilling. By the way the old Kenmore tank still works on the U-32000, though I had to replace the small rubber fill valve washer after 7 years of service (I replaced it with a small faucet washer from the hardware store).
"¢ Small fill cap on the tank. The tanks on some humidifiers I have owned have large fill caps, which have a large seal area, which makes them very hard to open/close and makes them more prone to leakage due to large seal area. This 1.75" ID cap is the perfect size.
"¢ Well designed "“ has interlocked water level sensor/ tank in place sensing micro-switch sensor. I have not taken the U-32000 apart, but the Kenmore 15123 had both an internal overheat sensor in the heating element, and an internal fuse for safety.
"¢ Long life "“ my Kenmore #15123 humidifiers lasted around 8 seasons with fairly hard (mineral deposit) water. The heating element finally stopped working after 8 years due to internal corrosion from the excessive mineral deposits from my water. I was surprised it lasted that long.
"¢ Replaceable "Absorption sleeves" make it easier to remove the mineral deposits from the heating element. I replace mine on first use every winter, though replacing it only once per year can sometimes be difficult since the mineral deposits attach the sleeve to the element very tightly, and can make it difficult to peel off. I also try to drain the base every few days or so just to try to remove the high mineral concentrated water. By the way "“ Kenmore 15110 absorption sleeves may also work.
Dislikes:
"¢ The humidity knob is very sensitive and may be sensitive to heat as well as humidity. Turning a little results in a large humidity change. However "“ since most other humidifiers have no adjustment at all, this is still a handy feature to have. If you don't care about maximum humidity then just crank it all the way clockwise. If you do care, then turn it all the way counterclockwise, then slowly turn it clockwise until you hear the soft click of the internal micro-switch (or see the "humidity" light go out), then set it by turning a bit more clockwise. I have not tried the newer model U-32010 "“ it may use a different sensor.
Suggestions for use - when you first receive it you might want to check the following to make sure it has not been damaged in shipping (though I have found no problems with the three I have owned):
"¢ Obviously look for any huge dents in the box. My box came with a small dent but the foam packaging seems to have protected the humidifier.
"¢ Look for any cracks in the tank.
"¢ Place it on a waterproof surface "“ I use a cookie sheet with a ½" lip around the edges. I have not any leaks yet "“ by why tempt fate.
"¢ Fill the tank all the way and hold it upside down (ie with the fill cap facing up) "“ there should be no leaks. If there is a steady stream then you have a leak in the tank and air will leak in and water will overflow the base if you put it right-side up into the base, so contact Optimus.
"¢ Then hold the tank right-side up (ie filler cap down) "“ you should only see a few drops come out of the fill cap "“ but not a steady stream.
"¢ With the tank out and no water in the base, plug in the AC cord, and turn the control knob to "low". The "refill" light should be on to disable the heating element. If not then shut off the unit immediately because the float switch is not working and you need to contact Optimus.
"¢ Put the tank in place. The water should gurgle out of the tank and fill the base. After a few minutes the base should fill and raise the water float, and the "Refill" light should go out.
"¢ Now rotate the humidity knob all the way to the left (CCW) the "Humidity" light should come on.
"¢ Now rotate the humidity knob slowly to the right (CW) the "Humidity" light should go out somewhere between ¼ and ¾ of the knob travel. Rotate it a bit further to the right so the humidifier stays on.
"¢ Now rotate the control knob to High. After 10 seconds or so you should hear a slight "shshsh" sound and fell hot moister rising from the top.
"¢ Now tip the tank slightly away from the heating unit so that the tank sensor switch on the side of the heating unit pops out. The "Refill" light should come back on to sense this and disable the heating element, so the "shshsh" sound should rapidly diminish.
Suggestions for use:
"¢ As with all humidifies - make sure to always fill the tank all the way to the top. If you only fill it a little, then there will be no vacuum in the tank to prevent water from overflowing the base, and you may end up with water on your floor.
"¢ Make sure the tank sits squarely on the base. Failure to seat it properly can result in leakage or failure to engage the tank-in-place-switch, or failure to engage the cap fill valve.
"¢ Drain the water from the base on every other tank refill or so (depending on how hard your water is) to lower the amount of mineral deposit build up on the heating element.

Before I give my reasoning for the poor review, I will say that this humidifier had a very good humidity output and the steam did not get horribly hot. On the downside, even when using filtered water, the buildup on the heater and sleeve was ridiculous, even after a day or two. The sleeve would stick to the heater, slowly wearing down the sleeve. The heater then started leaving black coloring on the sleeve. The sleeve was discolored within 2 days time. The manual says to replace your sleeve as soon as it becomes worn or discolored. After just under two weeks, even with proper maintenance, the sleeve was unusable. The manufacturer representative told me on the phone sleeves should last 3-5 months. He was not willing to help me in any way. I would not recommend this product because of this hassle and the fact that it will not last. Did I mention that the first one I got leaked and ruined my floor and I had to exchange it?
Edit: immediately after posting my review I discovered that the black "rubber" coating on the heater that the customer service rep assured me couldn't have been coming off on the sleeve was in fact worn off in quite a few places. I called back and they said they would exchange the product. I have no desire to have a third unit sent to me after two fails/two manufacturer defects.

Works great. I live at 8,000 ft and the air is very dry. I've been using it for 4 years now without complaints. I haven't even had to change the filter, I've just been washing it. I keep this one in my bedroom and I have a large blower fan humidifier in my main living area. The difference with this one is that it actually has a visible warm mist steaming from the top. This required me to move it away from a chair and wall or else it would get wet and moldy. The blower one in my upstairs doesn't have this problem - but it's also super loud and cost 3 times as much, so overall I think it's a good tradeoff.