Nice to have a few gallons with you AND have mobility. Both hands are free also.

More pumping than I had thought before buying, but now I understand that there's an internal chamber that gets pressurized, and it's just smaller than the 4g reservoir. So you pump maybe 10 strokes, then go about spraying. The stream can reach about 20 feet or so up. Enough to reach the underside of my eaves of my 2 story colonial house. Very handy for washing down your siding.

For a long time I owned a 1 gallon garden sprayer and used it for occasionally spraying the garden or lawn. Then I used it to spray Wet & Forget on my roof. My roof takes about 6 gallons, so it was quite the chore to haul the 1 gallon sprayer up to the top of a two story house six times. I bought this and it only took two trips up to the roof. It was also easy to mix an entire bottle of concentrate into the device, instead of measuring out smaller portions. The longer spray wand was also wonderful, as I don't like getting any closer to the edge of the roof than needed. The pump handle allows for continuous operation, which made the task go much faster. It was wonderful for spraying the roof. Then I did a spray job on the lawn to selectively kill the chickweed, clover, etc. that crop up every year. It wasso easy walking around with the sprayer on my back and the long wand so I didn't have to bend over.

As I said initially, this is our second one, I had been very happy with the originaly sprayer. I wouldn't have even had to order a second if I hadn't tumbled head over heels down a rocky, overgrown incline. Big bruises for the frame, bent the frame on the sprayer and apparently jarred a join on the sprayer. Glad to have the damaged one for parts.

The received this June 2015, it initially (first use) displayed a strange pump performance, swing back behind the sprayer making it difficult for a short person to prime Now the pump mechanism will not even pump up the sprayer. I am going to see if there is a warranty. VERY UNHAPPY.

Overall this sprayer first seemed solid and performed well. However, on the 4th,5th, and 7th uses the rubber plunger seal has disconnected from the pump plunger, requiring draining the tank, unfastening the pump assembly, fishing out the tip, and replacing it -- a messy, time consuming job.
It appears that the rubber plunger can easily come off as it is only press fit. Unless this is improved I wouldn't buy it again.

UPDATE 1:

Now the rubber seal on the plunger will come off after 10 minutes use, making using this sprayer a trial of frustration. I try to pump very delicately but to no avail. Downgrading to 1 star.

UPDATE 2:

Customer service quickly sent out a new plunger seal which was easy to replace (well, after my constant practice taking out the plunger...). I strongly suggest that when your seal does come off several times, that you quickly replace it with a new one rather than struggle with the old. I think it is easily damaged.
Another reviewer suggests that this problem can be avoided by leaving a significant amount of liquid in the sprayer and never pumping to empty.

Given that the problem occurred in the first place, but that customer service was helpful and quick, and the sprayer overall seems well made, I'm updating my review to 3 stars.

I'm finding the Chapin SureSpray a big help with my spraying needs. I now can mix up 2 1/2 to 3 gallons of spray at a time and dispense it easily as a fine spray or stream to reach the high foliage on our fruit trees. Keeping the pressure up is easy as the pump can be operated while you spray. I find the straps adequately comfortable and can load the sprayer onto my back after filling it by lifting it onto a table first; it sits upright on it's metal base frame without tipping over. (It's too heavy when full to attempt shouldering it from the ground). The spray pattern is very easily adjusted via a twist at the nozzle. The wand is long enough so that bending is not necessary to reach ground weeds. And the curved tip makes it easy to spray the bottoms of leaves in the garden without interfering with other types of spraying. I've had no problems with clogging or corrosion: The tip appears to be plastic. The pump works to pressurize a small chamber within the tank rather than pressurizing the whole tank as typical hardware store sprayers do. That means you get pressure almost immediately when you operate the pump and can keep the pressure up easily as you spray. Pumping is very easy although I forget to keep pumping sometimes so my spray pattern will then vary. Not a big deal. My only complaint with the entire unit is that it is uncomfortable against my bony back when it is loaded with more than a gallon; a bit of padding on the tank and a waist strap to keep the unit from shifting as you pump would both be welcome.