I'm having problems with lag, but I think dib8rman has had better luck. I had some playable matches, and with one guy it was completely free of lag. But other times is really slow, but not as bad as Smash Bros I guess.
The most annoying thing for me is finding someone to play, it could take more than 5 minutes to find a match, but maybe this can be solved with more people buying the game.
But still, I'm really liking the game a lot, I'm not too much of an online player and this has other options to keep you entertained.
Castlevania Judgment FC: 1161 - 3389 - 1512
3DS Friend Code: 3480-2746-6289
Wii Friend Code: 4268-9719-1932-3069