The image says it all. I took the leap to the Unity 5 beta since their CloudBuild service now supports it. They are on their 19th stable build and I've only experienced a few crashes.
auto upgrading is magic
Unity 5 will auto-upgrade your project if you allow it too. It will also update your 3rd party assets. This is sort of scary as I don't want to be in the business of maintaining all of these assets. This should get resolved as devs get around to updating.
The process of auto-updating took around 5 minutes for my reasonably small project. There were a few errors that I had to fix and some Editor tabs that wouldn't render. I had to completely reset my workspace layouts but no big deal.
only real issues have been shaders and lighting
Unity no longer doubles incoming light value in standard shaders! So all of your shaders that multiplied the light magnitude by 2 need to be updated. For me this was all of them.
Also I had to update the way I was doing atlassing as they removed Matrix operations from surface shaders. Not a big deal except that I wrote a tutorial on this. I've already updated my post from a few weeks ago with the Unity 5 way of doing it.
a couple days worth of work
Overall I'm glad I did this. The effort was small and I am able to start using all the new API's and tools. My bet is that migrating early, despite it being a beta, will be less effort than migrating later.