I backed this ages ago when it appeared on Desura. It's absolutely playable, but the lack of sound is a huge gaping hole that drags the rest of the game down with it, and I think the FPS isn't limited, so it runs pretty hot. Bear that in mind while playing.
There's not currently much to do, but there is character customisation, day/night cycle, squad recruitment, skill leveling (via use/rust system), visible equipment, faction relations, town hubs, guard patrols, trading/trade goods, some limited base/building construction system, and the biggest world map I have ever seen in any game, bar Daggerfall. 98% of it is sand and rocks, but still. Very few games scale the world realistically... this one does. The draw distance is mightily impressive- you can still see people moving around tens of miles away, even if they appear to be just a single pixel on the sandy horizon. Thankfully, to make up for this, you have control over the speed of time so you're not watching your guys travel for hours to the next city.
It also has an impressive health system. Damage occurs to different parts of your body, and the character's animations and gait accurately reflects where he's injured. There's lots of weapon types, and associated skills for using them. There's also non-combat skills, like medical and running and whatnot.
It's been a while since I played, so all of this is outdated knowledge, but it should give you a limited idea of what to expect. I do know that people have already started creating mods.