p.2 #2 · Best Wide Angle M42 mount (without breaking the bank)
There's also the Sigma 20mm 1.8. Seems to go for around $300 used.
The first one I had sucked but Sigma replaced it and the 2nd one is pretty good. Off the top of my head I would say that it's fairly sharp, probably sharper in general than the CZJ 20/2.8 I have but with more distortion. I like the CZJ better myself and it's a nice size.
p.2 #3 · Best Wide Angle M42 mount (without breaking the bank)
I know this is a shameless plug but I was happy with my Nikon 20mm f4 AI on my 20d. I've recently moved to the 5d so the lens is for sale ( with AF adapter) on the B&S. Some test shots can be found here: http://www.pbase.com/vickery/nikon_20_4
