I love the SP.
I never really got into portable gaming before. When I started to get really into console games, I bought a Game Boy Color - but the screen was only visible if I sat directly under a lamp or used one of those stupid worm lights (which only illuminated part of the screen). So it was really too much hassle to use. I got pretty excited by the graphics capabilities of the GBA - but soon after it was released, a friend & I went to one of the big Virgin Megastores in London, and although the GBA was sitting in a hands-on display unit designed to sell the thing, we could barely see the screen well enough to play. It was running Kuru Kuru Kururin, and I kept crashing the spinny thing into obstructions that I couldn't actually see! The Afterburner mod interested me, but despite the fact I live with an electronics engineer, I didn't really fancy buying a brand new games console then invalidating the warranty immediately. Plus, trying to find a spare £100 to replace a potentially fried system didn't really appeal.
So the SP was the answer to a lot of problems. I was pleased to see it'd gone back to the classic, "one-hand" shape rather than the must use two hands shape of the Advance (and I utterly *heart* games like Pokemon that let you set the L trigger to map A, so you can genuinely play one-handed). And although the screen is only front-lit not back-lit (and still a nightmare if playing outside in the middle of summer), it's orders of magnitude better than any of the previous GB incarnations. Being able to play a game anywhere has filled in a lot of the useless moments in my day - it amazes me that I spent 400 hours of my life playing Pokemon last year, but less so when I consider how much of that was in 15 minute bursts on the train. (What else would I have done during that time? Stared out the window? Daydreamed?). The best bit of all is that I can play games in bed, lying on my back if I want, without sitting up the entire night because I'm waiting to get to the end of a piece of plot.
I still walk around amazed that i'm carrying in my pocket a computer more powerful than the one I used for word processing and graph-drawing when I was a teenager. (It directly translates, too, because that particular Acorn computer used an early version of the same ARM processor that the GBA uses).
I'm reserving judgement on the DS. I've held one in my hands, and it's heavy compared to the SP - too heavy for me to easily be able to lie on my back and play it. Also, as someone with crappy joints at the best of times, I worry about the touch screen and new input methods (every apparent innovation in game control can be a PITA to someone with a disability). But... I'll probably buy one fairly soon after it comes out. For Animal Crossing DS, even if nothing else.