I've mostly been playing the "Play With Anyone" matches, as I haven't been fortunate enough to be brawling online to play with friends, for the most part. Unfortunately, those time that I have been on, those on my friends list are usually in other matches already. I wish that Nintendo allowed a simple ping or an invite so that you could get the attention of those on your friends list.
Anyway, I've probably played somewhere around 200 matches online at this point. Of those, I've only had two instances where the lag was severe, and probably 10 - 15 matches where lag was noticeable but not game-breaking. Normally, when I have experienced lag, the problem seems to lessen if I play multiple matches within the same group.
The biggest problem I have had is during the match-making. Sometimes, it's taken my Wii FOREVER to actually connect with people, and on occassion I've lost my Wii online connection during the waiting room sequence. This may be due to my ISP being flaky during peak use hours, though. I've experienced this problem with other games in the past.