I think I've seen one or two reviews from people that have actually tried it and say they noticed latency/lag, with dozens of reviews from people who haven't tried it saying that would probably be the case. The other reviews so far (that I've seen, I guess) have all been from people effectively saying they might not even realize they were streaming from the cloud if they didn't know otherwise.

Mine gets here tomorrow though, so I guess I'll wait and see for myself.