A few years back, I put a powerdyne on my '88 Mustang. I was easy to install and worked great. I had no problems on the street or at the track. The easy ones had problems with the internal belt breaking but I near had any problems with it.
When I got rid of the car, I sold the supercharger to a girl I met at the track. She still e-mails me once in a while to say her car is going faster and faster. The kit I bought was rated a 6 psi and I think she changed the pulleys to get 11 psi and hasn't had any problems.
a wee bit off topic, but a magazine had a 05 mustang magnacharger in testing, should be released soon for around 3500 w/o intercooler, think it as extra 500 for built in intercooler...3500 for a roots type blower setup is gettin pretty close to the cost of a few centrifugals, and supposedly give a LOT more bottom end torque...I want one, but momma says otherwise
(I still think thats an awful lot of money though when you look at what you are actually getting...very high profit items there.)