First of all, this is my first prime lens. I've shot extensively with zoom lenses, and really fallen in love with them. Their flexibility is amazing.
In order to gain a better appreciation for composition and some of the other fine points of photography and to simplify the number of decisions I have to make when packing a photo bag for a trip, I decided to pick up a good prime. I went with a 50mm because I think it is the most versatile for my lifestyle.
Another huge consideration was that the lens is capable of F1.2. This allows me even more flexibility because it gives great low-light performance. Nights out on the patio with friends and family, outdoor evening bbqs and any other low-light situation are much easier with a lens capable of doing well without a flash. Indoor photography is greatly benefited as well.
The two obvious questions I had with this particular lens were:
a) how is sharpness full open
b) does it really have the focusing issues that others have cited (back focusing being the most common)
I'm pleased to report that neither are an issue for me. Granted, I don't have the best trained eye in the world, and I haven't done any serious post-production work to really be able to tell, but so far sharpness is the best I have ever seen.
I'm using a Canon 30D body (for now), so I am not taking full advantage of this lens (I am switching to a FF model when the 5D replacement is available or when I win the lottery and buy the 1DS Mark III) but the ~ 70mm effective focal length is still very flexible.
I love this lens. It is the fastest lens I have ever used, razor sharp, and can literally shoot in the dark.
I was walking down the pier shortly after sunset, with decent moonlight and was able to capture boats in the water with no flash and in great clarity. I was amazed. In fact, I had to double check that my flash had not fired.
I'll have to play around with the custom functions so that I can assign a button to manual/auto focus switching, but the lens is capable of full-time manual focus, so for now I am mostly leaving it in AF.
The "feel" of this lens is fantastic, and I think the weighting is nearly perfect (I am running a Canon grip on the 30D). The focus ring is perfect in my opinion, and reflective of the overall quality of the L line by Canon.
I don't yet have any negatives to report.