I used this lens on the 20D and now on the 40D. The focal length is not ideal for the 1.6 crop, if you intend to do mostly portrait stuff, because you have to stand far away from your model. I use it not only for head shots (with that lovely creamy bokeh), but also for some tele applications, where a fast and relatively light weight prime is more handy than my big, heavy and white 70-200f/2.8IS.
The 135 produces very minor (almost negligble) Chromatic Abberations at high contrast edges when wide open and the sharpness is aready very good. Stopped down to f/2.8 the CA is gone and sharpness improves a little from almost excellent to absolutely excellent.
With the 135f/2 you get a sharp, fast, tele and portrait prime with a great cost/performance ratio (at least by L standards). It's simply one of the best lenses ever made by Canon.
Nevertheless I think I will be enjoying this lens even more, when I will be adding the next affordable Canon full frame to my camera arsenal (i.e. the 5D successor). But this is not the fault of the lens.