My copy of the Canon 50F1.4 is sharp and AF is fine at F2. I rarely shoot at larger apertures. In reading over this thread, it looks like many people have different lenses when they comment on their Canon 50F1.4. That is because they do. The digital picture did a comparison of multiple copies of the lens and reported large copy variation.
Some people have worse copies than others.
Contrast is not as good at F1.4 than F2. This is not a surprise.
The lens is sharp stopped down, and I can't tell any difference in performance between the Canon 50F1.4 and Sigma 50F2.8 macro at F2.8 and smaller apertures. Of course, color rendition is different.
I keep a hood on the lens at all times, even in storage when not in use. This is not my most heavily used lens, and so I can't comment on reports of fragility. I have no doubt that lots of people have had problems with this.
Personally, I would really like to see Canon come out with a 50MM F1.8 or F2 IS with USM. A new 50 F1.4 would be nice with USM, but I suspect it would be very heavy and expensive. The AF on my 50F1.4 is accurate enough, but not as fast as true USM.
There is no reason a 50 mm f/2 IS USM would have to be that big and heavy. After all, Canon has a 35 mm f/2 IS USM and it is fairly small and light weight. While the 50 mm f/2 would probably be a bit bigger, it probably wouldn't be significantly larger.