Have you played the demo yet?
I would suggest playing that, and then add a few pretty dumb NPCs hanging about the place, shooting shit up. There's also cooking, illness, professions (minor character customisation), curtains and craftable doors, and various little tweaks.
That's pretty much it.
It's certainly playable, but whether there is any more than 3 or 4 hours genuine playtime out of it is debatable. It depends on your attitude to the type of game as it is, you might find yourself frustrated with the lack of features, or you might enjoy the ones that are already there.
You can also mod the current build (as you can with the demo, I think) so if you are interested in this side of things, you'll get a bit more value out of it. There are some excellent map mods out there if you look hard enough *cough*
anything done by me *cough*
.
One thing to say though is that progress will be relatively slow in the next few weeks in getting new features in, so don't buy in to it expecting weekly new builds.
NINJAEDIT: Also yeah, good point from lordcooper there. You can't save, and I doubt you will be able to for a good few months yet. If you want an extended playtime, you need to leave your computer on and the game paused ... ... ...