Spending quite some time in the public transportations every day (~2h total), I needed either a lot of books or a good handheld.
The DS seemed to have interresting games, and above all two (overlapping) categories of original games and games that you could just pick&forget (open the DS, play for 5-10mn then put it down).
Started with NSMB, Kirby, Meteos, Phoenix Wright and Mario&Luigi: Partners in Time in the first batch...
All frigging awesome but M&L, which is a huge piece of crap (except that it's not even huge, 15 hours or so to get done with an ARPG is an infamy). Then went to Advance Wars DS (awesome, I don't play it that much though) and Castlevania, and I very recently discovered Otsu! Tatakae! Ouendan!
The latter is the best game ever. Bar none.
Oh yeah, MKDS is nice in multiplayer too. Average in single-player.
Right now, I'm trying to get hold of Age of Kings (heard it was even better than advance wars) and eagerly waiting for Yoshi's Island 2, the new Castlevania and the ARPGs that should be released by the end of the year.