My experience with online has been great thus far, but I should disclose that I rarely/never play games online, and I do not own an 360 or PS3, so I can't make the sort of comparisons that likely matter to those who are used to consoles that pump out smooth online experiences.
I've put in about 15 hours of online play so far. The frame rate does take hits every now and then (I particularly noticed this on the Industrial stage), but I've found it to be more than playable. If you are some hardcore nut to whom every kill and death counts, it's possible that this could be a cause of some frustration, especially when your aim at an enemy that magically jumps from one spot to another two feet ahead of them ends up being the difference between a win and a loss. There have been instances of this happening, but where my shot still registered. It's not perfect, but it's thus far been very fun to play.
There was one game that went haywire, where every time I started on the map, I was instantly killed by the same person, no matter where I was. This seemed to be happening to other players in the match as well, as people kept joining the match, and then leaving it. Bizzare.
I've also been playing using the Wii remote and nunchuck setup because I have not yet adjusted to the classic controller. The maps are great, if not just a touch too small. But it keeps the matches moving along at a brisk pace.