I play it regularly. Once every couple days at minimum. I play nothing as much as my DS, which I'm playing daily. Of course recently what I play most is my PC. Orange Box took over my soul. For me the order in which I use my systems recently goes: DS > Wii (+ GC games and VC) > PSP > PS2. Normally I would be using my PSP more than my Wii because I'm always playing some classic RPG or other, but currently the RPG games I'm playing are GC games (Fire Emblem and Tales of Symphonia).
PC is topping all of them right now. I certainly don't regret owning any of them. Considering I'll be purchasing at least 4 Wii games between now and December it will probably take top spot, though it's hard for anything to usurp the 30 amazing DS games I own (plus 2 more DS games coming).
