My brother picked up a DS on the launch date. I was at first (and partially still am) skeptical of it, mainly because nintendo still haven't provided a good use for the second screen with the current batch of games. However, playing games like super mario 64 on a portable system is a real delight, something which would be unthinkable only a few years back.
Other than the back catalogue, the next best thing has to be, for me, the touch-screen. Not only is it used really well in Wario Ware Touch, but my younger cousins can easily have a go on it without being overly complexed by what's going on. Case in example; they can pick up and play any stylus-based game with a few minutes of learning. It's that pick-up-and-go factor that impresses me.
Granted, i'm still looking forward to what the PSP can put onto the table...heck, with situations like this, it's always good for the consumer
Edit: I'm not sure whether suggesting that the upcoming GBA2 is a sign of nintendo's defeat...they've always said that the DS was to be a new development line, away from the GBA itself (hence, even before the DS was released, we were expecting to see two nintnedo handhelds on the market).