The specs are nice. Using one of the 44-pin A4 chips, you would get (I think): - Twice the flash (64K) - Twice the RAM (4k) - Twice the clock speed (32MHz) - Four times the A-D resolution (12bit) - PWM on all digital pins.
But there are significant "complications". The first is that this tentative design throws away some pin functions to get similar behavior to Arduino. On arduino, the rx/tx pins match up with different byte pins, so it is no longer easy to write a byte-at-a-time to D0..7, and it could get worse.
The thornier problem is the power supplies. the xMega operates at 3.3V, so VUSB is now regulated, and I'd just as soon use the usb jack a 5V in for external supplies rather than have yet another source. The "9V" pin could become useless, or it could get get VUSB. The 5Vpin should probably get VUSB, but COULD get 3.3V as the "main" voltage supply (since no shields will draw sufficient current from 3v3.) It's all quite a mess :-(