Huge improvement on my old 75-300. *Much* sharper. IS is excellent and works very well. I couldn't decide on this or the 70-200 L f/4. I glad I went for this - it is plenty sharp enough for what I want and the extra length is usful as is the IS. Focusing is a far bit faster than the old zoom but is still not exactly instant.
It also attracts less attention that the L glass as being black it looks more normal! Sometimes the white L teles seems to say "mug me" ;-)
If only it had the ring USM and ftm focusing :-(