For me, TrackMania probably comes right after the Tribeses and Battlefields for most hours spent in multiplayer. Terrific stuff.
I probably would never have bought it too if it weren't for the free version. That's honestly all you really need (almost everyone just plays on Stadium servers; it's got the best physics, best parts, best tracks), but I figured I'd support them by buying United. I'll definitely buy 2 on release day.