Summary: This is a perfect lens. The 20mm field of view is close to a persons normal field of view. What I see through this lens is more in line with what I am looking for as opposed to the 24mm 2.8. Do not get me wrong the 24mm is great, but the 20mm is more pro looking with its large 72mm filter thread and larger overall size than the 24mm with it's 58mm filter size. This can be a soft lens in the corners if you shoot wide open. Your field of view is curved so compensate.
Summary: I bought this lens with some skepticism because of some bad reviews. The truth is that I found this lens to be an exceptionally high quality lens, with great IQ and well built. It has become my walk around lens, I have learned to appreciate the wide view provided by this focal length. I use it with the Canon 5D Mark II, I a not sure if that is the reason of its excellent behavior. In any case, I love it and I recommend it.
I am so glad I bought this lens that's now the default lens on my Canon 7D.
7 May 2012
Summary: I use my Canon 7D mostly to shoot video. Mine came with a zoom that's totally mediocre and I bought right away a 50 mm 1.4 lens that works magic in low light situations. However 50 mm is too narrow in most situations. I needed a wider lens of good quality, that opens decently AND that I could afford ! This is this one. Got it for $220 on Ebay, practically new really and I love it.
Conclusion: Have had this lens since the mid 1990's when the street price was high. Yes there is corner falloff and vignetting in a 5D MKll, so what shoot around those limitations and you have one fine lens. Stacking filters is not a good idea. I get tack sharp results very fast autofocus, great low light performance and you can stick it in someones face and get a nice shot that isn't so exagerated that it looks comic book. Takes great close ups with great depth of field.
Pros: excellent close ups, great dof, light weight/small, very sharp