I remember reading a review on all 3 of the 50mm lens early this year, it concluded the 1.4 was the best buy, if you can afford the extra £200 over the 1.8, if you can't then you won't be disappointed with the 1.8. The 1.2 was fantastic, but is around £1000 more than the 1.4 - hence the money no object lens.
I picked up a 50mm 1.8 from Ian (Kerso) on here for a work colleague who was leaving and he has been blown away with it, he is very very pleased with the resluts he's getting.