I've had the RG421 with the Graphtech Ghost piezos in there for awhile, and the sound is pretty serviceable for the most part. But as per Aaron's advice and experience, I finally got around to buying the Fishman Aura Sixteen. The Spectrum is very cool too, but seemed a bit overkill for this application.
After playing with this for awhile, it's just not possible to go back to piezos without the Aura. It sounds absolutely amazing. Anyone with a piezo-equipped electric owes it to themselves to try one out.
I give it two weeks.