Regarding the “quality”:
Of course zplane’s or iZotope’s stretching can sound better, but their license models and CPU usage overhead are not acceptable for us.
Also remember that we’re using the stretcher in a sampler, and not for “simple” audio clip based playback as for example in Live. In a sampler every voice needs to be stretched separately and in realtime, with possibly changing pitch and tempo, so the audio signal never can be looked up. In audio clip playback, the entire sample can be preanalyzed first, which makes stretching a lot easier.
So in our realtime scenario, Rubberband IMHO really does an excellent job. But if that works for you, is something that only you can decide of course. And it’s an “option” only after all. You don’t need to use it. There always is the option to offline stretch a sample with other tools if all you want is resampling some loop to some other tempo.