I found that the upgrade to 2006 was uneventful (using a Knoppix live CD and the 2.0 flasher), including restoration of the 2005 backup, and the new OS to be stable, snappy and with so many little added conveniences it almost feels like an entirely new machine.
...
The only reason I can think of for not switching is if your daily usage of the tablet REALLY depends on some package that was available for the 2005 version and has not yet been recompiled for the new one (like Abiword, Gnumeric and some others).