The Pokemon series is the primary reason people dismiss the DS, and the reason people missed out on so much. (I'm guilty of this until recently.) If you're playing the DS just for Pokemon game, stop and look here.
The Pokemon games are far from the best. (D/P was awful, Platinum only managed to fix things. HG/SS feels slow and boring compared to G/S/C. B/W is hit or miss.)
The DS is my favorite video game hardware of all time, with only the Wii being close second. A one-two punch from Nintendo that managed to knock me out. Of course this is taking into account the Gamecube and Gameboy Advance backwards compatibility. The library it has is absolutely huge, and it had everything you need. RPGs come with most of the Dragon Quest series, Mario and Luigi, Shin Megami Tensei, and even the best version of Final Fantasy IV. You got some of the best visual novels including Ace Attorney, 999, Hotel Dusk, Trace Memory and Ghost Trick. Arguably the best Mario Kart. The best puzzles. Etc etc.
The library is only rivaled by the PS2. You can have an argument for days about which is the best of the two, but it comes to preference. The DS was home to the best 2D styled and innovative games while PS2 is more streamlined, 3D and homes great niche games if you're into those. Hardware-wise they're outdone by their rivals (PSP and Gamecube), but they completely curbstomped their competition with the library and the phenomenal marketing. Everyone owes to their selves to own one, if not both of them.
That said, I want to put my top 5 DS games here:
1. Elite Beat Agents
2. Rhythm Heaven
3. Mario Kart DS
4. Hotel Dusk
5. Need to play more of the games before I have to decide this one. Mario 64 DS might have gotten the spot if it weren't for the awful lack of analogue control.
Notice how my top two are both Rhythm games. I do want to mention that, before I played those two, I was absolutely against Rhythm games. (Still am for the most part.) But these two are the best games I have ever played in a while because of the simple yet challenging game design full of content.
Here's to the 3DS following the footsteps of it's predecessor. Wii U doesn't look so bright though.
Also the price... Wii U is $299.99 and you can get a PS3/360 with hundreds of Gigabytes of storage + Games or even Kinect for 360 for less than that.
The DS is full of memories for me. My first memory was finding that box waiting for me last day of school. I quickly unboxed, set-it-up and played a game on it and I instantly loved it.
My next memory is buying Pokemon Diamond and playing it. It was pretty lovely, considering all my GBA Pokemon games were destroyed/lost. That was... 4 years ago?
Then I got SoulSilver, which I absolutely adored (until I lost my Pokewalker) and White, but I wasn't very content with White. It was niche while it lasted, but the system was too outdated for lazy ol' Game Freak to put another generation on it.
Oh, also my cousin destroyed my jet-black DS lite so I had to buy a Silver one. It was a heartbreak.