The light alone made the SP tens times better for me. When playing the GBA, I either had to play with a window behind me or right under a light. Once you see what the screen looks like under a lamp, there's no going back to playing on the couch with a screen that yes, you can see but not like you should be able to. I pretty much had quit playing it because I didn't like having to find the "perfect" spot to play. I know people like to claim they have no problems with the GBA and lighting. But you can't deny that playing a GBA under normal conditions (avg light in a room) and under a lamp will make a difference with how clean and crisp the screen looks. It needs to be perfect for me.
Anyway, I traded the GBA for the SP and it's a world of difference. I can play on the couch, outside, on the back porch, in the kitchen, at the PC, laying in bed...it doesn't matter, I get the clean, crisp, bright screen every time. I've had it a couple months now and have no scratches on the screen because of the flip-down lid, another advantage. Personally, I don't play every day but I'll usually play 5 or 6 times a week. About every 2 weeks I'll have to recharge it.
As for the comfort, the face buttons are about the same to me as the GBA. The shoulder buttons are better though. With the GBA I had a hard time keeping my fingers on the buttons at all times when playing. Actually, I didn't because my hand would cramp up. Playing games like F-Zero was somewhat awkward. With the SP, my fingers naturally lay on them. Much easier. To the guy that said he couldn't find them....I have no idea how that is even possible.