Canon 70-200mm

I'm always on the hunt for a new lens. I shoot on a Canon 60D and the kit lens it came with is...nice... but not the greatest. I absolutely love my 40mm f/2.8 pancake but I think I might be interested in the Canon 70-200mm. 

There are a few versions of this lens out there, the cheapest of which being an f/4. Not the fastest, but the price tag is hard to argue with. Maybe if I can get a good deal on a used model I'll scoop one up. It would be nice to have that kind of range in my tool bag. 

Any of my readers use a 70-200? What are your thoughts?