Okay, the main issue first: it's definitely not worth spending more for the Canon-made version! This does the job fine, and a little hard-plastic ring is all any maker's version is going to be.

Once I figured out how it went on, I had no real problem with the attachment process: just face the filter-screw threading outward, line up the two parts of the guide mark at the side, then push on and turn 90 degrees (until the line matches up with the dot at the bottom on the barrel).

It does NOT form a complete seal, though, so you will still need to clean off your camera's lens regularly when dust or moisture gets in behind this.

The design has the circle divided in 4, with 90-degree arcs of attached vs. loose, and clockwise/counter-clockwise makes no difference when detaching. On my old camera (a Canon S2 IS, overdue for an upgrade!) there was a release button that had to be held down when detaching the adapter; the SX40 has no such lock or stop, so when I'm changing a filter I worry that I'll turn both layers, and set the adapter loose again.

I like the fact that the adapter attaches just below the tip of the lens barrel, and moves in and out with the zoom; my old S2 required a bulky adapter cylinder/cone because it attached at the base! I do rather wish there was some way to slope the abrupt transitional profile, though -- it sometimes feels like you stuck a saucer on the end of the lens barrel, when you're trying to edge the "nose" in somewhere. I actually accidentally "popped" the filter assembly off once when I moved the camera too hastily back from a window frame, and was just lucky it didn't break anything. More plastic wouldn't add much weight, but it would mean a larger item again, a nuisance if you're carrying it in your bag rather than attached.

Anyway, if you want to add accessory lenses or filters to the SX40, though, that's the way it goes. The design is what it is, for good and for ill.

The particular Vivitar specimen I received has a little molding dimple or "hiccup" in one edge, but it does not affect either the latches or the screw threads.

UPDATE: I found one important negative/risk with having it attach to the end of the lens barrel. I didn't always use to make sure to turn the camera off when cleaning and reattaching the adapter-plus-UV-filter (which I leave on as a lens protector), but once when I went to push it back on, the extended lens gave a bit under the pressure -- I'm crossing all my fingers that I haven't stripped the gears! It seems to be OK so far, but I'm not chancing this happening again, and the damage becoming cumulative. (On the S2, no force was required -- simply align and turn, and it clicks solidly into locked position, so this wasn't an issue. ) LEARN FROM MY MISTAKE, and always turn the camera off first, so the lens will be fully retracted!!!

UPDATE 2, November 2012 -- It just broke. That's within the year, so I'll see how the warranty replacement process works out.

I read a lot of complaints by others suggesting that anything made to adapt to the Canon Powershot SX40 HS didn't fit well, couldn't be put on, etc., but that has not been my experience at all. I purchased this adapter ring so that I could add a polarizing filter to my lens. As soon as I got it I put it on the camera. It went right on, with ease, taking only a half turn to snap into place without forcing. When I received my filter I attached it to the ring easily, a couple of seconds and I was ready to get outside and start shooting. My filter was secure, as I let my camera hang from my neck while I got into position for the shots I wanted to take, I had no problems. It has nicely textured areas for easy grip, this is important to me as I experience some tremors. I would recommend this product to anyone with a camera like mine.

I wanted to put a UV filter on my camera since I do a lot of hiking and find myself in dusty environments and didn't want to damage my lens and this was a necessary adapter for my Canon PowerShot SX50 HS. Though it is a lesser quality than many others, it serves its purpose and allows for the filter that I wanted to be easily screwed onto the camera. The quality of the inside rim that hooks onto the camera is not made as well as the rest but it doesn't really have to be to do its job. It is a little tricky to get it in alignment, but that is a small problem. If your looking to save a few bucks for something that doesn't affect the quality of your shots, this is a good buy.

This is made of plastic which I was looking for and what I liked so it won't mar the lens of the camera.
If you go by the metal kind and it is a tight fit, then you already know which one will give (the lens and not the filter adapter ring).
This plastic adapter ring won't damage your camera but stays on it quite well with just the right amount of tightness.
The finish could better but is good enough and won't be noticed once you have a filter on it.
This is a good copy of the genuine article although not an excellent copy that you can't tell them apart.
I just gave it 4 stars for the finish (how it has been molded or made)..as for the fit, it fits quite good and firm.

Got what I paid for! It does the job on my SX520. There are a couple of things one should be aware of though:

1. The adapter is extremely difficult to attach to the bayonet mount on the camera lens. Be careful and patient and it will fit ... otherwise you risk buggering it up. Then one has to decide whether to leave it in place always: I would recommend leaving it on.

2. Since the cameras in the Canon SX line are small bodied, this adapter ring will cause you 2 serious problems because it extends beyond the original diameter of the lens. First is that it will partially block the focus assist lamp to the upper left of the lens. Second is that it will block some of the bottom of the flash field and you will see a shadow on close shots. One has to determine whether being able to use filters is worth those problems.

3. Filters tend to 'stick' in the adapter. Use a little bar soap on the filter threads to ease removal later.

4. The adapter is plastic and easy to damage if one is not careful. The adapter only comes in plastic, even from Canon. This adapter is exactly the same as the one from Canon that costs 4 times as much. There is a metal adapter available like some reviewers have stated, but the quality doesn't seem to be there.