BUYER BEWARE (12/30/10 update):
I have received several comments on my review (Thanks, James!), have read several other reviews and have looked at the pictures posted by other users. Please be aware that the most recent pictures and the some of the most recent purchasers have NOT received the same product that is pictured. Please review the pictures that I have posted compared to several other users. If your results are significantly different than mine using a Canon EOS 18MM lens (or equivalent), then it is possible that you received a different product. If you received the same product, there should not be significant distortion in the picture or black rings around the edge when using the same lens at 18MM.
I purchased this lens attachment in lieu of purchasing a brand-name wide angle lens. I wasn't expecting much, since it was only $10. That said, I was pleasantly surprised by the results.
I tested this attachment with my Canon EOS XSi and an EF-S 18-55mm lens, fully closed to 18mm. I shot images with and without the wide angle attachment, and noted a clear difference between the width of the target area. I viewed the images side by side on my large monitor (32" LCD HDTV) and did not see a significant loss of image quality. There was slight loss of sharpness, but not much.
Using the macro attachment with the same lens, I noticed a clear difference in the minimum focal distance of the lens, but I did not notice any loss of picture quality. The close up image was very sharp when viewed on the computer screen.
In both cases, autofocus did not work properly, so I had to use manual focus to achieve the best pictures. When holding the shutter button down halfway, the green focus indicator within the view finder worked properly though, and helped me get the best focus.
There are clearly higher quality lenses that serve this purpose, but this lens (wide angle and macro) is definitely something I will carry in my bag until I can purchase those more expensive lenses. It is definitely worth the $10, if you need something now.
NOTE: I also added pictures showing my test results. Follow the "See all # customer images" link below the main product picture to see my examples.
NOTE 2 (9/11/10): I've received a few questions about this lens and wanted to qualify this review a little bit. Shoppers please remember that this is a $10 lens. Even off-brand lenses for the Canon cameras costs $200+, so please do not expect to achieve the same quality results using a screw attachment. It may work in a pinch, but I would not rely on this attachment for critical photo ops when you can afford a true wide angle lens. Despite my initial thought that I would get a lot of use out of this lens, I haven't taken it out of bag, since I wrote this original review in December 2009. I haven't found the need for it. If fact, for wide angle or panoramic pictures,I use my camera's continuous shooting mode to take a series of pictures and use Adobe Photoshop Elements 7 [OLD VERSION] to splice the pictures together. It includes a feature that does this for you automatically and the results are amazing.