I just received my AVR ONE! and thought I'd share my initial thoughts of the device:
The main device (the big silver box) appears to be quite well built; it is smaller but heavier than I anticipated, which is nice. However, the plastic connector (probe) that comes attached to it seems like it will be quite easy to break; the torque of the JTAG/ISP adapter when pushed was alone enough to cause the black plastic female header to fall off (wasn't a problem putting it back on). I'm not sure this will last very long, and since it is required to perform level conversions and costs 100 dollars to replace, if it goes, I'm SOL. Drivers installed automatically in Windows 7, and AVR Studio 6 had no problem connecting to the device. JTAG and ISCP programming is fast and reliable, but I notice now that HV Programming is not supported by the AVR ONE!, so I will need to hold on to my AVR Dragon afterall. I have yet to try out the Nexus trace functionality, but I'll revise this review when I do.
All in all, I think the AVR ONE! is a nice device, but it's definately a shame that level conversions aren't performed in the body of the device and a simple ribbon cable could be used to connect to the target. Furthermore, unless ALL programming modes are supported (which they aren't) I would have to say that the AVR ONE fails to live up to its name of being ONE device to do everything.