I picked a mint copy of this lens up about 2 months ago. It begins to impress right out of the lens trunk! Every time I shoot with this lens, it amazes me over again. Most of that amazement comes from comparison. The 70-200IS is a staple lens for me. Yet, when I shoot both at the same event, I can typically pick out the shots from the 300L right from the filmstrip view in Bridge. To me that takes quite the quality piece of glass. Thus far, I have really only used the lens for indoor sports, so I haven't seen the superfast AF that people rave about. That's probably because the gyms I've shot in are so low light and if I'm off target there is no contrast to pick up. The times I've goofed around with it outdoors, it seems to be very snappy.
Without the tripod collar, I find this lens to be very comfortable to handhold. I tested out a Sigma 120-300, and it was much more cumbersome to handle. From looking at the specs on this lens, I don't have to tell you that it's going to get heavy rather quick.
So far, my only minor quibble with the lens is that the hood is so huge and obtrusive. It's hard to find a bag to hold this lens ready to shoot without spending a small fortune. It fits nicely in my Tenba DB17 backpack, but limits the other items I can carry. Most other bags geared to carry this lens with the hood cost as much as the backpack I use.