NES: has stopped working, looks kind of dusty, but still retains it's greyish color that it had when we first got it (used) 18 years ago.
GB: still works perfectly, though you can kind of tell where I've been holding my fingers/thumbs on it over the years.
SNES: still works fine, dusty, still plugged in to the smallest TV in the house (or apartment, since I live across town from my parents).
N64: also works fine, needs dusting, plugged in to same TV as SNES (they're brothers afterall.)
GBA: I can't seem to find my original GBA.. I think I gave it to one of my little cousins who had lost his at school (he's a Mario fan too.. I had to help the boy out!)
Gamecube: other than loading games a little slower than it used to, it works fine. Needs a bit of dusting, but has been unhooked from the big TV at my place (made room for the Wii).
GBASP: looks like new. One small set of dead pixels (or maybe it's just one dead pixel) close to the bottom left corner kind of distracts during Mario Kart, and the hinges are more loose than they used to be. Otherwise fine.
DS: Touch screen needs a good cleaning, Stylus seems to slide out of holder a little easier than it used to (like somone said above), and GBA slot needs to be cleaned out. Otherwise fine.
Wii: There are some very faint scratches along one side of the system, but they're not from paint.. I was carrying my laundry and a little black bag (with the Wii and all cords/controllers in it) down my parents front porch steps, and I completely missed the last step and fell straight down. I landed on my knees, and the laundry basket cushioned my fall, but I heard this lound crunch right beside me. The Wii bag had hit the ground pretty hard. After resting for a little bit, I took it back to my apartment to look at it: there was a very strange black marking on the side of it. I cleaned it off with a damp cloth, and then tested it to see if it was still working. EVerything worked fine. Wii are durable! :D
It should be noted that all controllers for all of these systems work like new (except for the N64's, which has a bit of buttons get stuck from playing a lot of Mario Party or SSB).