If you are interested in PH and Mario64, you don't have much choice but to get a DS!
I really think the DS has great games for those 1/2-hour "pick up and play" sessions, especially between commutes. That's where I use the DS most.
About the games, these are ones I'd recommend (because I've tried them out/finished them and found them to be highly enjoyable, some borderlining addictive):
- Meteos: Fantastic block puzzle game, and really highlights the use of the stylus - you can play the game without the stylus, but you won't get anywhere near the scores you'd get with one.
- Tetris: I'm pretty sick of the traditional Tetris (think first-generation) but the DS version has a few quirky and unique gameplay modes that keeps Tetris fresh
- Elite Beat Agents: If you like music/rhythm games, this is a must! For import, Ouendan, although I found Ouendan to be far more difficult because I don't understand Japanese and the lyrics get in the way of the beats.
- Mario Kart DS: If you have a couple of friends with DSes, multiplayer is fast, furious (and frequently frustrating but still) fun.
- Castlevania: Dawn of Sorrow: Great platformer; I found it much more enjoyable than Super Mario DS. It was also the game that broke the DS (literally; my DS mainboard broke from pressing "Jump" too hard, too many times). I also think this one is better than Portrait of Ruin because PoR kinda "cheats" with the game's completion rate, but that's another story.
- Advance Wars: Dual Strike: I love turn-based strategy games (Civilization, UFO/X-COM) and this game gives me the fix I need. Also to keep in mind if you like this genre is that Civ is heading for the DS.
- Mario & Luigi: Partners in Time: Very enjoyable RPG. I liked it much more than Final Fantasy III for the DS.
- Tony Hawk's American Sk8land: Very cool cel-shaded game that is easy to grasp but unfortunately a bit short. Otherwise, very fun and the punk skater music complements the game perfectly.
I actually own pretty much all the "greens" from the Metacritic site ( http://www.metacritic.com/games/ds/scores/ ) but these are the ones I feel most people would enjoy without reservations.
Some other recommendations are a bit touch-and-go: You may like them, you may not, but this is very much dependent on the genre:
Point-and-click adventures (in the vein of Monkey Island or Space Quest):
- Phoenix Wright
- Hotel Dusk: Room 215
I've also heard that Planet Puzzle League is a blast. I plan to pick it up as soon as I finish Hotel Dusk.
Bleach is surprisingly a solid game as far as portable fighters go. I don't think it's available in English, though. Daigasso! Band Brothers is also a fun rhythm game but it's also in Japanese. However, neither game requires Japanese reading skills to play.
I have finished both Rub Rabbits (or whatever it's called) and Feel the Magic: XY/XX, which I thought was great for short bursts of playing but admittedly it's a very "Japanese-audience" game.
What the DS absolutely sucks at, up to now, is in driving games. Apart from Mario Kart DS, evrything else (Carbon, Ridge Racer, etc) are flat-out horrible IMO.
You can also pretty much forget about FPSs, although I've also heard Call of Duty on the DS is surprisingly fun and really pushes the envelope in terms of what the DS can do.
Anything else in particular I missed that you'd like to know? I've played well over 100 games on the DS - there are about 15 coworkers that have DSs and we constantly swap games so we have a good bit of DS mileage accumulated...
