It's still a great little system. It has a large library with a lot of great games (like you said), but also quite a bit of shovelware. The phat and Lite models have backwards compatibility with Game Boy Advance via Slot-2 and a few DS games can interact with the cartridge in Slot-2. There's also the DSi models which sacrificed the second slot for better hardware and new functionality. The DSi has a few exclusive titles that don't work on the originals.

There's also a lot of cool homebrew for the DS family. For phat or Lite models you'll need a flashcart (I recommend a DSpico as the hardware and software are fully open source). With a DSi it's possible to softmod the system to run homebrew right off the system SD card.

An important thing to know is that the hinge is a bit of an Achilles's heel for the system. The plastic of the shell is prone to becoming brittle or even disintegrating over time. Regardless of which model you pick it's a pain to replace the shell. Additionally, only the Lite has high quality replacement shells available, so if you have a phat, DSi, or DSi XL then you're kinda out of luck.