If I try to just find random players, I'll never get more than one opponent. If more than one of the spots lights up, it either drops all two or three or (rarely) tries to start the game, resulting in an error. This is annoying, as a one-on-one game is vastly different than a match between three or four players and gets tedious much faster, especially when you're much better or much worse than your opponent.
Friends games, on the other hand, seem to work without a problem. I've never had enough people on to participate in a four-player match, but have had three on several occasions, with great success. VOIP also works flawlessly. Hopefully Nintendo will be able to get their server issues straightened out so the online experience is a little more consistent.