Smoother than butter on a baby's back. Like I mentioned before I really enjoy it, sometimes there is a bug or disconnect, but in terms of online connectivity this game blows smash bro's brawl out of the water.
The connection has gotten a lot better since the beta. I can play for hours and have problems maybe 2-5 games. I have even used my galaxy nexus to tether its 3g connection (seems more reliable than 4g for onilne gaming), lag free.