Doesnt seem too much different than gba in size and durability. I like to protect the screen with something most of the time (just in case) and I take very good care of mine. The D-pad seems a bit too sensitive though.
Well, I dropped my GP32 from not very high off the ground and it busted the R button!! I had to open it up and put it back together. Overall, I would say it is not as well built as the GBA, but a heck of a lot better of a game system.