Just got this lens and only taken a few shots so far. Firstly, it focuses faster than a lot of reviews suggest - it's certainly quick enough for me, especially on the 1D II. Manual focusing is lovely, smooth as silk and better than expected. However, the 45 point focusing a bit of a hinderence especially close up and I'm only certain I've nailed the focus when using a single central focusing point - honestly the DOF is THAT shallow! However, when you Do nail it, this lens is sharp enough to cut yourself with and makes my other L lenses seem second rate, even my beloved 70-200 f2.8 IS is put to shame! It's so sharp that if you intend to get any closer than a half-length portrait, you had better use very soft light or be prepared to spend a lot of time in Photoshop - it picks up every pore and every skin blemish! Goodness knows what it could do on a 1DS II!
The colours it produces are wonderful and it's fun to use as the DOF and bokeh are unreal. I spent months deciding whether or not to buy this and now that I have I'm having the most fun with my photography than I have since I was a kid. If you're considering this lens, do yourself a favour and buy it!