Reviews I read before I bought this lens in November 2008 said the 70-200 F4 IS USM is the sharpest zoom lens that Canon makes. Since then the 70-200 F2.8 IS USM ll has been released and it may be sharper. I use the F4 IS USM the most. I use it with the 1.4 Extender to take BIF pics on my 7D. I got great pics on my 40D with this combo. I am looking forward to this spring and summer to get BIF pics with my 7D. As it takes 67mm filters I can use them on my 17-85 too.
I bought this lens because I didn't think Canon would be updating it anytime in the near future. I felt they would update the 2.8 version next. I was right. I think it was updated in 2010.
I believe the F4 and the 2.8 have the resolving power to keep up with the new sensors. I am quite happy how my lenses
take pics on my 7D I am happier than when I just had the 40D. Raw files look better on the computer straight out of the 7D.