I’ve been using 3.0 on my iPhone for a few weeks now and have yo say – in the main – it is very stable.
Issues I’ve had:
The icons for some of my apps lost their icons
With iTunes 8.2 beta I couldn’t sync with my existing library when I upgraded iTunes ( release appears better)
Very occasional lock up on Pin screen.
All new features work well but with Beta 5 tethering and MMS disappeared :-(