My friend (Not me, and I want to very clearly express this point ) had a classic DS with a flash cartridge. Shortly after getting it he let me borrow it. It had nearly every DS game in existence.
The ones I spent the most time playing were 358/2 days, Elite Beat Agents, and Crystal Chronicles. All absolutely superb games. I regularly see people bash the DS for a lack of good games, but, I really beg to differ.
Plus all the homebrew on it. Really an amazing and flexible device. I mention homebrew because there are quite a few really good homebrew games. You'll have to look those up. haha.
I have around 16 Nintendo DS games in my draw right now. I don't really use them anymore, but since I'm a collector I won't sell them for years to come. Ever since the Nintendo 3DS came out my current DS just sits there and collects dust. Kind of sad if you think about it.
I don't have my DS any longer (gave it to my sister) but I only had a few games on it. Off the top of my head I had: Final Fantasy III, Rocket Slime, Children of Mana, Brain Age, and Castlevania: Order of Ecclesia. I didn't really play on it when I had it so some of my games were still in the shrink wrap. I just kept them in their cases otherwise I'd lose them.
I own a good amount of DS games ever since I got the very first one. I have about 20 something games that are still in their cases. I lost a couple of them, but most of them are still there. I stopped playing the DS after I went to college because of the lack of time. My little cousin has the 3DS and I don't feel like it is anything special, especially for the price you have to pay. You can get a good tablet for that price.
My kid is the one with the DS. She has maybe 15 games for it because both of us prefer mature console games like Grand Theft Auto 4. All the 13 and 14 year old kids I know prefer M rated games like Saints Row the Third.
So even people who are the right age for the Nintendo DS find it too childish. Go figure.