I have used Reaper's own stretching, and it is ok. Also if you are after varispeed effect (slowing down and at the same time lowering pitch, or speeding up and raising pitch), Reaper's rate control works nicely. With Reaper you have to test different algos and find the best for each situation.
BTW I know this wasn't what you asked but if you are interested in extereme slowing down while still getting good quality sound without irritating artifacts, check out Paul's Extreme Sound Stretch:
http://hypermammut.sourceforge.net/paulstretch/
That can make some psychedelic stuff!