My reading is that it is the final game (essentially the review build) so you could potentially finish it prior to the 1.0 release - and if not, it's fully compatible with version 1.0 as far as saves go.
Ah, I actually meant the base Early Access build. I wasn't actually a kickstarter backer, and don't even own the game yet. But if I could start it in earnest without worrying about missing anything, I'd probably buy it now.
EDIT: Oh shit, just read on the early access rundown that they're planning on adding more sidequests into the final version. Guess I'll just have to wait, because I'm assuming that includes Act 1 and the beginning of Act 2