Like many, I was considering this lens along with the Canon 35 2.0 and Sigma 30 1.4. Really just trying to replace the feeling of a 50mm lens on a film body. One can go crazy trying to find the perfect solution here, because there is no perfect solution - all of these lenses have their problems, whether it's CA, build quality, corner sharpness etc. You just have to know what you want out of it and go with the one that gives you more of what you're looking for.
For example, a common complaint about this lens is CA. And yes, if you point this lens up into the sky to photograph tree branches you're going to get some CA. But is that why you want this focal length? I use it mostly for people shots, and lots of indoor available light shooting. CA isn't a problem in these situations.
Looking at samples from all lenses, I don't think there's a huge difference in IQ. The Sigma may have slightly better center performance, but in real life situations (i.e. non-pixel peeping) the difference is barely visible. I find this lens' center sharpness at 1.8 to be pretty good, though I may be in the minority on that one. It improves a good deal stopped down even a little bit.
I've found AF on this lens to be very fast and very accurate in low light. Build quality is as good as it gets for non-Ls - on par with the 100 2.0 and 85 1.8.
The only downside really is price. ~$400 is a lot for this lens, especially considering that the 35 2.0 can be had for around half that, with similar speed and IQ. For me, though, the difference in build quality and AF speed is worth the extra cost - to others it may not be.
All in all, a great little lens. It does what I want it to do and excels in most situations. Highly recommended.