Rather than repeat all of the good things everyone else has said, I will focus on my opinions of the negatives everyone else has said. This is based on my experience of using it as a primary walk-around lens for the past 2 months.
1. Slow AF. Yes, its true, but I found it is not much slower than my 50/1.8, and honestly, it hasn't been a terrible issue for me. I frequently shoot moving targets and my OOF rate is about 2/3. Frequent pre-focusing helps alot. AI Focus can help too.
2. Weight. Yes, its heavy, but its also not as bad as most people make it out to be. Its a short lens, so the weight is close to the camera body, so it is actually not so difficult to hand-hold and keep steady, especially compared to longer lens. The way the lens frame narrows towards the rear makes hand-placement for holding rather convenient. I once walked around an event for 6 hours with this mounted on a 350D w/grip and 580EX all around my neck. No problems (but you need a comfortable neck strap!).
3. Usage. Many will say this one is best used in a portrait studio where it can be safe and secure on a tripod. Bollocks. There is no disagreement that this lens may have little equal in that setting, but the idea that it should be held captive within, rather than allowed the freedom to go out, is sad. It can do so much more. In my case, this stays on my 20D as my primary walk-around candid lens. I now prefer it over my 35D and 135D. Granted, one must accept the challenge of the slow AF and razor DOF wide-open, but the results are worth it.
4. Care. It does require a bit more conscious thought. Canon strongly recommends it be set to infinity for storage so that the lens elements are fully retracted. The rear element is quite flush to the metal mount, making it high risk for scratching if carelessly handled while attaching/removing from the camera body. Not a bad practice to have anyhow.
Overall, I have been extremely happy with this lens.