Well I shifted to 3.5 and have regretted it ever since.
The mistake I made was not testing it enough first, if I had I would not have upgraded. Too many errors, perhaps not all down to bad code, but perhaps down to architectural design and low support for backward compatability. I know the beta has been running for three months or so but this doesnt mean the new shape is a good fit for what plugin developers might consider to be an enabling evolution. As moderators for WP are quick to point out when looking inwardly, plugin and theme developers are more often than not voluteering their time to create free product for others to use, and perhaps WP if too heavy handed with design changes might be closing the door to a lot of them!
Main thorns for me personally as a user so far that have come to light are:
Editor: not working correctly.
Images: (disaster) with Set Featured Images - none are displaying this alone means I am looking now to see how to rollback.
Of course I am not saying they are not fixable, but I have not even had a response to a thread I started about the images issue - hence look at if I can roll back. And I like the developers have other fish to fry, I need it to work without to much overhead.
Tell us if your experience has been good or bad?
Personally - Bad.