I was too lazy to put an Afterburner on my son's GBA, and the GBA SP wasn't thrilling enough to get, so I planned on getting the Nintendo DS for my kid as a Christmas present. Pre-sales sold out of these fast everywhere, but I correctly guessed that when I headed out of the metro DC area for Florida for Christmas, I'd have no problem finding one in retirement-land. Nabbed one at Best Buy for $149 two days before Christmas.
Luckily I brought his Flash2Advance cartridge with several GBA ROMs along, since the DS games offered so far are prety lame and the demo can only do so much. He much prefers using the sylus instead of the thumbpad so far, and had a great time instantly networked with another kid at school playing the demo against each other. He's happy so far and probably will be still 6 months from now, even if only playing GBA ROMs on it.