Pretty nice for a first one! Decals look hi-quality, and it seems that at least a bit more than 5mins has been taken with the skin. Some tips though:
1) Skins are stretched a bit when imported to the game models. This should be taken in account when skinning. Some info concerning this: http://www.lfs-gear.com/TUTORIALS/TuT_Stretch.html
2) You could have tried to continue the squares/triangles from side of the car to the front/back. (Tough one, but the skin would've looked much better.)
3) Exhaust pipe shouldn't be white.
4) If you aren't already using Master Skinnerz kits, they're a great help http://www.lfs-gear.com/kitz.html