My concern wasn't so much the way the game plays online, although Nintendo doesn't have the best track record with online play either. Smash Bros. is super laggy for me with anymore than 2 players on the screen at once.If the Testfire proved anything it's that Nintendo was able to handle a lot of people on their servers at once. I played 2 of the 3 hours and I don't think I ran into any problems at all. I was concerned about that, but after the Testfire I think it should be good.
My concerns are two things when it comes to the online play. Number one is communication, the Wii U has pretty limited voice chat functionality as it is and they never have implemented communication online very well in the past. The other concern for me is finding and playing with my friends online. Unfortunately with Nintendo being so far behind Sony and Microsoft in terms of online functionality, I just don't expect much from Splatoon.
Don't get me wrong, the game looks like a lot of fun and I am sure it is. I'll definitely pick the game up, but I may wait for a price-drop first, Nintendo is making a big push for this game, but I am not sure that the core Wii U audience is looking for this sort of game right now. Definitely glad Nintendo is trying something new with Splatoon though and I hope it does well so the trend will continue, Nintendo is in desperate need of some new IPs.