I bought my flame GameBoy Advance SP this weekend.
Nintendo has fixed the biggest problem with the old GameBoy Advance, the unusable screen. The screen on my GameBoy Color is better than the old Advance.
The SP is smaller than the Advance. The folding design protects the screen, while making the device more compact.
The SP screen is a thing of beauty. The switches in the control pad feel better than the older GameBoys. They are apprently using some sort of microswitch rather than the conductive rubber buttons.
I can't believe how good it is. All my old GameBoy games look better on it.
Using headphones requires an adapter, a bit strange. That's the worst thing I can say about it.
Oh, by the way, the new Pokemon Pinball Ruby/Sapphire ROCKS!