An alternative to Canon flashes are the Metz units. I have a few of them myself. Less money than Canon, ETTL compatible when Canon version purchased, and more modes of operation. They do have the ability to fire when other flash units go off which was always a good way to get around the limitation of 3 flash groups when using optical wireless mode. For the exposure times I use during portraits, and other indoor shots, there really is no lag time issue, though I'm sure at higher shutter speeds it would become an issue at some point. Haven't felt the need to test this further to see what the limit is.