I'm a fan of the Canon 70-300IS too. It's small, light and produces sharp images. The only things I don't like about it are the front element rotates and the colors sometimes need enhancement during PP. I used it along with a 135L and a 200L yesterday at a horse show and you could tell quite a difference in the colors of the 70-300 and the "L"s straight from the camera - not much after PP. It's a great vacation lens and I use it quite a bit for a "walk around" lens.
Here's a pic from yesterday's show taken with the 70-300.
I've also seen posts about it being soft at 300, but that hasn't been the case with the one I own.
Here's a shot I took while on a vacation in Florida
1/500s f/5.6 at 300.0mm iso200
Canon 7D, 5D, 35L, 50 f/1.4, 85 f/1.8, 135L, 200L, 10-22, 17-55, 70-300, 100-400L, 500D, 580EX(2).
Sigma 150 macro, 1.4X, 2X, Quantaray 2X, Kenko closeup tubes, Yongnuo YN685(3), Yongnuo YN-622C-TX. Lots of studio stuff.
** Image Editing OK **