Just upgraded my n810 last night.
Saved my settings / apps, reflashed to diablo and installed my settings / older apps as prompted by the upgrade process.

Some hiccups along the way. For example all the original mp3s, desktop patterns and document files reappeared (not too much of a surprise) but I ran out of space when reinstalling my apps.

Some weirdnesses involving the Modest app wanting to be upgraded ubt failing (I've fixed that now)

Some weirdnesses with application hangs (I had to pull the battery twice now)

The GPS with AGPS does seem to work MUCH better for me.

Reinstalled Debian, works okay once I installed the extra hildon packages which seemed to get lost in the upgrade. Symptom was that the IceWM window wouldn't go to full screen.

Things seem a little wonky still, I find the repository management and application upgrade process pretty clunky.