lol, no way the 85L keeps up with the 50L. Especially hate it when you miss focus with the 85L, it keeps going and going, then goes reverse and on and on. I didn't want to believe it when I read about the 85L being a one trick pony, but it really is. Other than posed shots, or predictable lateral movements, there's not much it can do. I do have it permanently mounted on my 5D so that I would try to learn it more, but it's fairly upsetting using it as a normal lens.
I've used it successfully to shoot volleyball. It can't switch players to get the pass, set and hit all in a sequence, but it can follow one player at a time just fine.
It's not a fast focusing lens, which means acquistion is slow. It does not mean the lens cannot follow action at all.