I'd say it was the best toy I've every bought my ds (dd1 and also dds2). My ds (5 and half) got his for last christmas, although he played with his sisters gameboys from when he was 4). Definitely buy the sp version as the screen is bigger and illuminated you don't need to buy anything else as it comes with a plug that re-charges the gameboy (no need for rechargable batteries).
The only thing I'd say is restrict the amount of time he spends on it and not in the evening as I think it affects his ability to 'wind down'.
Oh, I've just remembered you need to get him some earphones - the noise does tend to drive you mad - although it has a volume control, so it can be turned off - but ds seems to like to music!!