Personally I've played d3 maybe 40h, and it was only on launch day I had problems. I'm satisfied with the performance, and I like cloud storage of my game stuff...

Of course I have 10/100 persistent fiber to home connection. Dunno what I'd say if I was on a crappy connection.

Also I've LAN'ed D3, but we did have a pretty decent connection + router then. No NAT issues or such reared their ugly heads. At least they got some of their network code right.