Christian is right. On any forum you will hear more about problems that people have than about things that go well...
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 2006 AppCatalog IS growing quickly and already thick enough given that the final release is barely a fortnight old. And at least these are properly packaged and managed...
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).
Myself I skipped the beta period because I didn't want to be without Privoxy, but as soon as it appeared I jumped the gun and have not regretted it one second since.
And obviously, all the interesting new apps, or upgrades of existing ones like Maemo Mapper are going to be 2006-only from now on...