I've never tried PalmGUI (well I don't have USB, either), where is it available?
I still run Spitfire with an older Palm IIIc and a serial link. It's excellent! Palm and Amiga make a good combination for ppl who can't take their Amiga everywhere.
AFAIK, Spitfire wasn't updated to work with any USB stacks. It's a shame really since I've been using a Zire72 for last several years, too.