brundlefly
SONAR generally gives pretty reasonable results with the right material stretched by a reasonable amount, and rendered with the right algorithm. But I understand there are DAWs that do it better.
Based on the DAWs I've used, which is pretty much all of them, the best algorithms are iZotope Radius (used by SONAR) and zplane elastique Pro V2 (used in current versions of Sound Forge, Ableton Live, Ueberschall, Cubase, and some others). Note that zplane also licenses several "downsized" versions, like elastique soloist; I'm talking solely about the Pro V2 version, which is their best.
I don't detect any significant difference between Radius and elastique Pro V2 in terms of sound quality, or I'd export audio from SONAR for stretching. There have been instances where one sounds just a teeny bit better than the other, but it's not predictable which one will be "best" (depends on the program material), and the difference is so small it's not worth taking the time to stretch and evaluate.
Of course this applies only to offline stretching and rendering. Real-time stretching is something else altogether, and Acidization/REXing can be very good or very bad depending on the editing, the stretch amount, whether pitch transposition is involved, and whether you're speeding up or slowing down.