Basically, imagine having a regular cab s 10 pickup. That's about what you can fit in a Fiero in the the way of stereo, but if everyone hates you and you don't get groceries I've seen the passenger side traded in for a fiberglass dual 12" box.
As for the vehicle itself, they are poorly made. The paint on them ******, and in the winter you spin around like a top. The cooling system is in between insufficient and non existient. Working on the motor is a *****. The way they are balanced is not anywhere near a BMW, and it cant be exactly classified as a handling machine. The front end slides out around improperely banked corners (you can't steer, um, kind of a problem).
But there are bunches of awesome body kits out there, and with one properly installed they look sweet as hell (Ferrari look alike kits etc). Plus, if your're huge like me, you can fit into them, because the footroom on them is nice.
Basically think of it as a sort of a small white elephant type of a vehicle. You can't do much to them in the way of super performance, but you can end up with a good looking, fairly quick and somewhat reliable vehicle. The only reason I never bought one is because mainly it's a GM product, and because there is a ****load of them around here.
Tell me about the one you're looking at, I'll tell you what I think.