Many decried the original GBA's reflective LCD screen and its reliance on external light sources. The Game Boy Advance SP's main feature--optional built-in lighting--fixes this complaint nicely. The quality of the backlighting is very good; it's bright and clear when looking directly at the screen, though less so from other angles.
The SP comes with a power adapter that recharges a built-in lithium battery; our informal tests found that it takes about four hours to fully charge (you can play it while it's plugged in and charging), and the battery lasts about 11 hours with the backlighting constantly on. Using the backlighting less often will conserve battery power.
Size, power, and affordability don't come without trade offs, though. There's no headphone jack here--you'll need a separately sold adapter--and the system isn't very loud at its highest volume. The L and R shoulder buttons are a fraction of the size they were on the GBA, and thus are harder to hit, and the reduced size of the SP is slightly less comfortable for adult hands than the GBA. The cartridge port placement on the lower part of the console is fine for GBA games, since they are flush with the console body, but older Game Boy Color carts will stick out in a way that takes some getting used to.
Overall, the SP is a significant improvement on, and seems just as rugged as, its predecessor--which has proven to be very rugged indeed. --Porter B Hall