I bought two 37mm wideangle converters for my Canon HF100 and wanted to see which one is superior.
Here are the specs and pros/cons:

Canon Wide Converter WD-H37 (NOT WD-H37C) .7X , 37mm, no front filter thread, $129.95
Pro: focus throughout zoom range, no softness on left or right sides, no vignetting
Con: no front filter thread, less wide angle

Century Optics Wide Angle Adapter .55X, 37mm, 43mm front filter thread, $124.95
Pro: front filter thread, very small length & diameter, more wide angle
Con: focus only up to half of zoom range, soft focus on 1/8 left and right sides, some vignetting at widest angle

Test Criteria:
Still image Mode, AV Mode, Evaluative AE, Cloudy White Balance, image size 1920x1080, Mounted on tripod, perpendicular to brick wall, no direct sun, bracketed exposures to get images as close as possible, photoshop resizing (no post processing or sharpening).

Century Optics 1/360, FL 4.8mm, F1.8 IMG_0263.JPG
1/90, FL 4.8mm, F4.0 IMG_0264.JPG
1/30, FL 4.8mm, F8.0 IMG_0269.JPG

Canon 1/360, FL 4.8mm, F1.8 IMG_0281.JPG
1/90, FL 4.8mm, F4.0 IMG_0282.JPG
1/30, FL 4.8mm, F8.0 IMG_0285.JPG

Here is the photo (half size) from the Century (f4.0, 1/90):
http://i30.tinypic.com/2n0rdip.jpg


Here is the photo (half size) from the Canon (f4.0, 1/90):
http://i29.tinypic.com/2nsqs78.jpg


The following pictures show left, center and right 100% views. For comparison F values of 1.8, 4.0 and 8.0.


http://i28.tinypic.com/idgnkx.jpg


http://i31.tinypic.com/nzpv7q.jpg

Some findings:
For both adapters: F4.0 appears sharper and F1.8 (not surprising) and F8.0 (unexpected).

On the Canon, F8.0 is blurrier than F1.8 (unexpected).

The Century has blurry left and right sides (about 1/10 of width or so) that is very noticable at f1.8 and not too bad at f4.0.

Bookmarked! Thanks a ton! Looking at your pictures, Canon's f/1.8 does not seem blurrier than f/4.0 to me, on contrary I like f/1.8 better but maybe this is just me.

In regards of f/4.0 being sharper than f/8.0 this may be caused by the camera itself, there should be a reason why Canon designed built-in ND filter of the HF10/HF100 around f/4.0. The HV20/HV30 is designed around f/5.6.

After my testing, I am trying to decide which adapter to keep. I love the small size of the Century, but I'm not happy with the image quality/zooming capability compared to the Canon.

I want to use a lens hood to keep out stray light. The Canon has no outer filter threads, so a hood is not possible.

I tested a 43mm lens hood with the Century and I get some vignetting. I tried a 37mm lens hood and attached the Century to the outside filter threads of the hood. Ergonomically looks compact. However, it caused massive vignetting that even shows up on 50% max zoom (probably due to the distance from the camcorder lens is now much greater.)

I'm going to go on a vacation next week and will take both.

I reported a while ago that I found F8 and F4 not as sharp as F1.8 (which seems sharpest ). And that was without the lens so that seems consistent with my results.

I did test WD-H37C and it's works great until about 60%-70% zoom. Then you see gradually softness and even CA.

There's also a vimeo video on century optics lens and I noticed same thing as in the screenshots. You do see at full wide-angle corner softness (which I think is fairly normal with 0.55x). But you also see more CA. I guess trade-off with increased wide-angle coverage.

I don't think it's a 'focus' issue meaning you could fix it by changing focus. It's rather CA and softness.

The biggest problem with the Century is that at around 50% zoom, you loose the ability to focus completely. It just becomes a big blur. I know I should just remove the adapter for zooming, but that can be a pain in some situations.

At least on the Canon WD-H37 you can zoom up to 100% and still focus. I got the WD-H37 on Ebay for $70, which seems like a bargain.