I had my dad pick up a few DS games I wanted when he came over to visit, and here're my thoughts about them:
Advance Wars: Days of Ruin - It's an amazing game that showed me a genere I was completely oblivious to. I have beaten all the campaign levels except for the last one, which is ridiculously hard compared to the rest (yes, even Waylon Flies Again, IMO). I haven't tried playing online yet but I like how the game lets you play with several humans on the same console (and that works like a charm since it's turn-based). I guess I might be pickung up other entries in the franchise later, or perhaps I could take a peek at Fire Emblem.
Mario & Luigi: Bowser's Inside Story - A fun game that reminded me how well the mechanics were built around the GBA's extremely limited controller. I've always found action commands a lot better than fixed combat in most RPGs, the story is okay and the graphics are gorgeous (I love how much personality they gave Luigi in most cutscenes). I've currently stopped playing right before the final boss, but so far it has been fun, if a bit linear and with uninteresting backtracking when you want to find extra stuff.
Castlevania: Order of Ecclesia - I've loved playing through it twice (a Shanoa run and another one with the unlockable character). I thought it'd be shorter since most areas are somewhat short and linear, but the final stretch had an amazingly long and varied level. Backtracking was never boring since most often you'd have new glyphs to clear a path (I wonder what were they thinking when they came up with the Nintesco glyph, though), and most areas were interesting (except for some palette-swapped ones)
Kirby: Canvas Curse - It's stylus-centric mechanics were great, although I'm still halfway through the game. Medal collecting might be a bit of a pain, but the rewards system is quite fun and you might be forced to play the game in interesting ways to fullfill the challenge criteria. I'm a bit puzzled as to how few games there have been that used the stylus as more than D-pad substitute; there were certainly a few released near the DS's launch (like Pac-Pix, Yoshi's Touch & Go and Canvas Curse), but I don't think there've been any relevant ones later on the console's lifecycle.
I couldn't get my hands on some less mainstream games then, but I guess there's always time to pick them up. I'll probably be purchasing a WiiU and/or a 3DS soon, so I might pick up less DS games next time.