They just announced a release candidate (or I just heard about it) and spent a bit of time this evening checking it out. The way I do that is by putting together a demo app that does something basic, that we already use Bootstrap 2 for, to see what the differences are.

http://bootstrap3.smallpict.com/modalDialogDemo

That's the "modal demo" -- sort of the Hello World of Bootstrap testing.

I think it looks nicer than Boostrap 2, which of course it should.