Jogger: if youve got an 18-50/2.8 it makes more sense to get the 70-200/2.8.. the 50-70mm difference is nothing and can be made up by walking. the extra 225-300 on the tele end is far for useful.
Sorry, but I have a situation where this lens may really be useful. I take a lot of theatre pics and with my Canon 70-200 on my 7D, I can't get wide enough to cover the width of the stage even with starting to back out thru the doors into the lobby. I can use my 17-55 at dress rehearsals but during the show, I have to shoot from the back due to the shutter noise and the 70-200 is just a bit to much telephoto. A 50 to 200 would be great, but I don't expect a constant f/2.8 version of such a lens will fit into my budget. :-}