Yeah, I used to. But I decided I like regular Minecraft enough to leave it vanilla until there's full mod support.
I especially love the Aether Mod and IndustrialCraft, they both add a massive amount of content to an already content-rich game.
Now that the next update will make sure the multiplayer and single player actually use the same code instead of the entire game being written twice, with twice as many bugs (wtf Notch? I'm really glad Jens is in charge of Minecraft now), we will see most mods going full multiplayer, which is going to be amazing.