I've used the R2 Beta with my original Droid and an AADS-XM system with the default Systembuilder Mobile Pro G GUI. It works fine. The Systembuilder Mobile Pro GUI is quite simple and does not provide things like multitouch scrolling or gestures. So I'm not sure how good of a test this GUI is for the R2 software.
If you are interested in testing for yourself you can join the Beta by emailing:
[email protected] This gives access to the software as well as an online forum where testers are reporting their experiences with the R2 app.