I bought one and returned it. Although the integration with the stock system is great, the sound quality was terrible. I suspect this may have had something to do with the included BlitzSafe adapter.
In any case, PhatNoise showed Phatbox 2.0 @ CES. It is expected to be available in Q3 2004. Additional features include WiFi transfers to your PC when your car is parked in your garage, iPod integration, CompactFlash socked and the usual HD cartridge.