First let me explain my negative comments. I have spoken with several people that are disappointed with some aspect of this lens because they really didn't understand what they were buying. Instead they made their purchase based on an "everyone else loves it, so I better get one" mentality. Save yourself a lot of heartache and dollars by making sure *you* need this lens before you buy it. It is big and heavy, make sure (rent one if you have to) that this lens will "fit in" with your photographic style.
Enough with that, now on to the good stuff.
This lens has, hands down, the most impressive image quality I have ever seen. Unless you have a trained eye (i.e. you know what you are looking for) images from this lens equal the 135/2L at the same apertures. When you see the pictures from this lens you will realize why Canon has not released a new prime since 1999. The bokeh this produces at f2.8 is something you have to see to believe. I have yet to do any contrast or saturation post-processing to its pictures.
As I said, second to none. I've never seen a better built lens. I've seen equals, but none better.
Yes, some switches could be better placed / harder to switch to avoid accidental switching. The feel of the zoom and focus rings are just divine. The lens hood does its job, and seems very secure on my copy.
All my lenses probably shared an equal amount of "on the camera" time until I got this lens. This is now my de facto standard. If you need a lens with these specs, and can live with its size and weight then you can get no better. But again, there are three very good alternatives (70-200 2.8 non IS / 70-200 f4 / Sigma 70-200 2.8) so do yourself a favor and do a cost-benefit analysis on all four of these lenses before buying any of them. Needless to say, I LOVE this lens.