Just jumping in here but I tend not to use the 3DS on platforms but I find it nigh essential on action/adventures like Ocarina of Time and Kingdom Hearts. I never put it up very high but being able to estimate how close an enemy is and how far away an object is is actually a big boon that I don't think a lot of people understand until they see it for themselves. I played Ocarina of Time and went back to Mario Galaxy and had an absolutely horrendous time gauging when to jump on enemies because that additional depth was missing.

Which is to say - great for games that exist in 3D; pointless for games that are already designed for 2D experiences.