I've used PT's built in one (sucks horribly), an older version of
Serato (on OS9), "Amazing Slow-Downer" (download only, a bargain at
$45, and usable within limits without even paying at all) and SoundToys
Speed. Speed was the best sounding by far, and ASD sounded a lot better
than Serato (though in fairness current versions of Serato may have
improved a lot). My vote would be for Speed, which sounded very clean,
even with pretty stupid amount of it, and also is the only one (AFAIK)
that has the ability to graphically draw in speed changes. It makes it
possible (though potentially very labor intensive) to correct rushing
and dragging in individual parts or entire mixes in a way that was not
previously possible in any user-friendly way. This feature alone would
be invaluable. The great sound seals the deal for me. I'm planning to
get it soon unless someone alerts me to something even better.

Ted Spencer, NYC