I'm currently using this lens (Tokina 50-135mm). I had the Canon 70-200 F4L IS but sold it away after getting this lens which is only 1/3 of the price. Yes, the Canon is superb but the Tokina is really good value!
At f2.8, pictures are fairly sharp as long you maintain a fast enough shutter speed. F4 is super sharp, almost comparable to the Canon.
I do event shoots so the IS doesn't really help with subject blurring due to movement as compared to have a F2.8. But if money is not an issue, pfft, I'd go for the Canon 70-200 f2.8L IS
For now, the Tokina fits my bill. Oh, it's build like a tank too...