I love almost everything about this lense. AF is not what I expected based on reviews here. I shoot a lot of motorsports and rodeos and my 70-200 f4 tracks much better on AI servo, it just has trouble locking and staying there. I use it on manual AF with great success. 2.0 is awesome for low light, just keep an eye on the depth of field, it's really thin.
Unless you shoot something with a lot of eratic movement that needs to be AF tracked, this lense can't be beat.