I bought the Canon 70-200/4.0 L USM
together with my 10D and am absolutely happy since then. Unfortunately I don't have a converter (yet), but Tim's pictures really make me want to put that on my Xmas shopping list
I am travelling a lot and therefore was looking for a relatively light (ca. 700g) yet solid lense. Having used an all-manual Canon AE
-1 and a Minolta Dimage 5 before, I am absolutely astonished about the very fast and reliable auto focus of this Canon lense. Definitely wouldn't give it away any more!
Please find below two similar samples, one in bright daylight and one in low light conditions early in the morning, both with relatively low shutter speeds to see the props turning: