I've had a 10 player MC server with MP for a couple of weeks now. The experience has generally been very positive, given that I was running a server of my main PC. The fact that they run off SSD's mean that the game is quite smooth. I am rather happy with the experience so far vs running the world on my own PC.
Lag is pretty much non-existent atm, although you have to bear in mind that our 'world size' is rather small when compared to Windle/OCUK's world and of course we have just 10 players. So far, lag hasn't been a problem aside from a friends monster spawner, which makes me stutter like mad when I go near it.
Updates - Agree with Windle. MP uses a server wrapper by the name of hMOD, which I believe has now stopped receiving updates from it's author hey0. They are due to move to the superior Bukkit wrapper at some point 'soon'.
MP use a 'Clan Forge' control panel for server management, which I am not too keep on. It can be slow and cumbersome despite having good features.
Customer Support - No idea yet, but their Minecraft forum has a very active bunch of mods/techies who can all help you out with any issues.