The Fiero is not a real powerful car, by today's standards. Back in the day it wasn't too bad. Can it be fast? Well, of course. The market for a Fiero is not real high, but is unique. The limit to what motor can be swapped is limited by your pocket book. The stock engines were rated ~98hp for the 4 cylinder cars (and get 30-35 MPG), the 2.8 V6 is rated at 140hp and 170 ft. lbs. of torque in a car weighing around 2700 lbs.
With a stock 3800SC, they can be high 12 second cars, with a few mods they can dip into the 11's. There are quite a few LS1, LS2 and even one with an LS7 (yes, the C6 ZO6 engine costing about $14,000). Northstars are common, and almost all other GM motors.
Reliable? That depends, like any other car, on how it was cared for. My 87GT has over 223,000 miles on it. A Fiero with over 200k miles is pretty common.
The little Fiero is gaining interest. The newest one is 20 years old and it still looks like a modern car. Search ebay and autotrader.com. The good ones are priced quite high.
A couple of examples of what can be done to a Fiero.
http://youtube.com/watch?v=7PDRZ0BJntE
The one below is the one with the LS7.
http://youtube.com/watch?v=bXF8GvhcMrQ&feature=related
See it built here. The LS7 appears on P.32.
http://www.fiero.nl/forum/Forum2/HTML/069871.html