use to think its great and its still is but you have to factor that the auto focus will die on you at times and under-extensive usage. It cost $150 to repair out of warrantly.
Alot of people don't talk about it but I have repair 2 50mm f1.4 lens 3 times in 4 years and I have move on, most of canon 50mm lenses are of poor value expect the 50mm f1.8.
50mm f2.5 half marco. Old type design and construction. not cheap and never owned it to comment.
50mm f1.8, hard to get quick focus, like TS mention. I got this problems until the eos1 series solved this problem to a great degree.
50mm f1.4, best value for money but prepare to spend maintain this lens.
50mm f1.2, super overpriced comparing to the 1.4 version, has focus shift focusing issues sometimes as reported.
50mm f1. Super super overpriced collector lens. I have never owned this lens to comment.
I still own the 50mm f1.8 and the 50mm f1.2 and i'm using the later lens more often for work purposes. There are other choices like others mention, sigma 50mm f1.4, if you dun mind manual focusing, I also owned a Carl Ziess 50mm f1.7 (cheap lens) for personal usage. There are actually lots of choices out of canon, some reportedly use leica R (out of production) lenses with excellent results.