I think the most important thing is to get a guarantee ahead of time, something like "if it chatters within the first week, we'll do it over for free".

The only time I paid a shop to change a clutch, it ended up chattering ... and so I said I'd always do it myself after that. Athough once when I myself changed anothe cluth the same thing happened -- even though I put in new crank bushing, new disc, new pressure plate, new throwout bearing and re-surfaced the flywheel. I suspect the pressure plate had a defect.