I used it about 50% of the time when I first got it, back when I was playing Kid Icarus and Pushmo. I find myself using it much, much less often now -- maybe 10% of the time. Possibly the novelty of it has worn off, but I don't think it's quite that. The 3D really makes the colors pop, and I do really appreciate how nice everything looks with it on. But it drains the battery faster and it's just a bit distracting. Sometimes I feel like it takes longer to follow on-screen action in 3D, making 2D much preferable for action games like Monster Hunter. But in some games, the added depth perception and sharper visuals are worth the tradeoff -- I don't think I could play Sakura Samurai in 2D.
Basically, it depends on the game. I used it for the cutscenes in Fire Emblem, but not for the gameplay.
