From my experience, not that much.
I have 2 original GBA's, one of them had the screen crack and all the LCD dripped out. (Impaled by a pokemon cartridge, or all things)
Other one has a huge dent in the screen, is scratched to hell, and the batteries have to be taped in tightly to work, otherwise they just fall right out. Not to mention it looks bad, dented all over the place, one of the corners is in bad shape.
My SP is also scratched to hell on the outside, the screen has some dust underneath it, but it otherwise flawless... For a while my R button wasn't working that well, but a nice, VERY hard wack against something and its working a good 90% of the time. (Well enough to play everything) But, overall, my SP has held up much better than either of my GBA's did.
I know i abuse my systems, i play the hell out of them, mainly at school. And while at school, the amount of shoving in and out of pockets tends to scratch them up quite a bit, lots of dents happen from shoving them into my backpack, or perhaps playing it between classes and running into something and dropping it.
Either way, the PSP looks like something that i doubt i could have last for 2 months. I'm glad nintendo makes their systems so damn sturdy, and i hope this thing is as sturdy as an SP. Electronic Brick was an awesome design in my mind.