I just purchased a Canon 70-200 2.8L (non-IS version) and am very impressed and it performs beyond expectations! Generally, zooms do not perform as well as primes and this becomes particularly evident in astrophotography where slight optical defects are clearly visible in stars.
This particular zoom however gets very close to prime performance. At 200mm it has similiar sharpness to my 200 2.8 L prime wide open, loosing marks on some of the brighter stars showing elongated rather than round halos. At 135 f2.8 it is even better edge-to-edge, with just some CA on brighter stars. Tests at 70 and 100mm show good performance wide open, but focus was marginally out and I need to retest.
Here is my preliminary test:
http://www.pbase.com/terrylovejoy/70_200
Terry