I've been running a production app on Edge Rails (straight from SVN trunk) for about a month now and it's been all good. The new-ish REST routing enhancements are great once you get your head around it. I also like the ideas behind the performance enhancements, though I haven't had a chance to really flex them yet.

Beware though, they "yanked" out a lot of common things from rails core, so if you are upgrading an existing app you may spend quite a bit of time getting things working again.