The Canon EOS and Photography Forums

Ken Rockwell has a review up on the 50mm 1.0. What a lens!
http://www.kenrockwell.com/canon/lenses/50mm-f1.htm

It's big, heavy, ridiculously expensive (~ $4000 used) and hard to find. The EF 50/1.2L USM (http://www.amazon.com/exec/obidos/ASIN/B000I1YIDQ/bobatkinsphotogr) is smaller, lighter, faster focusing, much cheaper (~$1200 used or ~$1500 new), easy to find and the optical difference between f1.0 and f1.2 in terms of background blur and DOF is very small.

My take is that unless you are obsessed with lens speed, want something unique for the sake of being unique, have to shoot in almost total darkness or have a bottomless pit of money, you're much better off with the 50/1.2L.  Canon dropped the 50/1.0L for a reason!

If you want maximum background blur and a fast lens, the new EF 85/1.2L II USM (http://www.amazon.com/exec/obidos/ASIN/B000EW9Y4M/bobatkinsphotogr) will give you more than the 50/1.0L, plus it generally makes a better portrait lens on full frame. It's also 1/2 the price and much faster focusing.

In fact even the cheap 85/1.8 will give you almost the same amount of blur on distant backgrounds as the 50/1.0 does when used for portrait work.

See http://www.bobatkins.com/photography/reviews/canon_ef_50mm_f1-2L_usm_review.html

I wouldn't say "no" if someone wanted to give me a 50/1.0L, but to be honest I'd rather have the $4000 to spend on other lenses. I love fast lenses, but the 50/1.0L just isn't worth the money (at least not my money!).

My "superfast" lens is a Konica 57mm f1.2 mounted on an Olympus PEN E-PL1. I wish I could put it on my EOS bodies, but the back focus is too short for focus at infinity. in fact you can't focus out to more than about 1ft with it on an EOS 5D. It would focus to infinity on an EOS-M (http://www.amazon.com/exec/obidos/ASIN/B008NENG1A/bobatkinsphotogr) though...now there's a thought.