I have one.  When it works I love it.  Not saying that there is anything
wrong with the ATI Procharger itself, it's getting it in your car perfectly
alligned so there wont be belt issues that is the problem.  I'm almost an
expert in serpentine belt issues on C5's.  I've had it for about 30,000
miles.  The car has 90,000 miles on it.  It's time to rebuild the bearings
in the procharger.  I haven't gotten around to pulling it off the car and
mailing it.. 20 degrees around these parts.. a little chilly to spend a few
hours in the garage.  Otherwise the power is ridiculous.  My 335's 18's
won't hook up at all at any speed.  0, 10, 30, 60 mph-- it doesn't matter--  
you can burn rubber.  BFG drag radials are definately the way to go with
procharger applications.  Just don't drive in the rain.  Would I do it
again?  Hell yea.  The oil is super easy to change.  Unscrew a cap and suck
it out with a plastic syringe.  Put new in.  Cost about $5 or $7 per bottle.
You buy it straight from ATI in batches of four bottles I think.

-Stan

From what i can gather it rally depends on the unit, some are much
loude than others.

mine tends to be much more noticeable at idle than when makeing boost.

i have a D1 with intercooler on my C4, and have had no issues with it
at all, after i got everything correct on the installation. there
install instructions were a bit off and i ended up pressurizing the
block under boost and that took all the seals out of the motor.

 i am not a big fan of the self contained units, they tend to run
very hot, from what i have seen and read, i prefer the ones that feed
engine oil to them and back to the pan.

the are a lot of people that have been had great results with the
Vortec units from Greg Carrol at "blowerworks" he uses water/meth
injection in place of a aftercooler.

Another great source of info is the "Corvette Forum"

There are a couple of books out there that you might want to pick up,
one is "supercharged" by Corky Bell and the other one is about
supercharging by A. Grahm Bell

Greg N