I have the P4XD and I use it for home practice / putting down ideas etc. It's obviously not a 'live performance' kind of effect, and the effect parameters are quite shallow indeed as far as 'tweakability' is concerned.
That being said, the sound quality is very good, it does not market itself as an uncoloured practice tool like the Cafe Walter, but despite the sound being processed, it is a good sort of processing.
Lots of usable tones and most importantly portable and easy to use. Great for bedroom practice before you sleep. Quick tip...get a 4.5v adapter if you're using it a lot. The old Sony MD and Discman ones work just fine.
Hope this helps.
J