For about the first 50,000 miles of ownership it was super reliable.
The only thing I can remember major going is the fuel pump a couple of months after I bought it.
Soon after hitting 100,000 things began to wear out such as the radiator, front suspension, alternator, heater core, window motor, lock solenoid, various switches, clutch cable.
The Mustang specific problems I have had are my fuel injectors started leaking at 120,000 miles, the pins in the door hinges wearing out and it developed a small crack in the floor pan which I had promptly welded up.
As of right now the only things it needs is a new rack and pinion and shocks and struts.
Also in 1988 the 5.0 LX models came with the very cheap base seats. They look like they came out of a Tempo or Escort. They are not that uncomfortable, but after 13 years and almost 150,000 miles the driver's seat is beat. I am pretty much sitting on the floor and the seat back creaks.
In the late 80s the 5.0 Mustang was one of the fastest cars on the road. With the exception of the Buick Grand National you would have to spend close to 3x as much to match the power of a 5.0 Mustang. People have to remember it is a cheap, light car with a V8 engine.
Yeah the body has some creaks and people complain about the brakes. The car is a little over powered for the suspension and brakes (easily fixed through the aftermarket) but if you don't drive like an idiot it is not a problem. The car was designed in the late 70s and I am sure they never dreamed it would end up with 225 hp and 300 lbs.-ft of torque ten years later.
I have been extremely happy with my Mustang. I consider it to be reliable and cheap and easy to fix when things do go wrong. For being a "muscle car" it is very good on gas. I have been able to get 29 mpg cruising at 80 mph. City driving it gets down to about 16 mpg. Having 2.73s might have something to do with that. Also there is a lot of aftermarket support for the 87-93 Mustangs. I don't think you will ever have a problem getting parts for these cars.