[danso]

I've dabbled with toy apps in 4 and have been very impressed with how smooth the change has been from 3 to 4. I haven't tried directly migrating a Rails 3 app so far (as 3.2.15 has been holding up quite well) but would anticipate it being far less of a hassle than moving from 2 to 3. AFAIK, Github is still on Rails 2

[Axsuul]

Upgraded a pretty big Rails app from 3 to 4 a month ago, only took an afternoon.

[kingnothing]

Github is basically on their own branch of rails at this point. They use the 2.x base, but have pulled in pieces and parts of subsequent releases.