The H1Z1 website reveals that Sony's zombie game is coming to Steam early access on January 15th (thanks Massively). SOE's John Smedley shares more on what to expect on reddit, making it clear this will still be a work-in-progress at that point:

After a lot of hard work, we've decided to make the release date January 15th, 2015. This gives us just enough time to polish what we have and put out something we'll be proud of and hopefully you'll be excited to play.

Why did it take longer than we originally thought? It's a fair question. The simple answer is once we saw what we could have had earlier it wasn't good enough and we wanted to put something out that we would be proud of. What you'll be playing January 15th is a really fun game, but it's early. It's everything Early Access should be - a fun and compelling game that isn't finished. Our goal is to have you help us finish it and have it be a game we can build a large community in over time.
Is it going to be a finished game? Absolutely not. If that's what you're expecting, DO NOT BUY EARLY ACCESS. The goal here is to let you in early and help us mold it into a game you want to be a part of for quite a long time.

We'll have 2 SKUS when we launch. The basic early access sku for $19.99 - which will let you into the game and have a great time, and a more expensive one which we'll announce here shortly. The difference between the two will be things like access to some of our different game mode servers (which you can get access to by purchasing tickets or earning them as well) and other specifics we'll be outlining soon.

Is H1Z1 going to be better than Day Z day 1? No it won't. We'll get asked that question a lot and I wanted to be up front about it. We're not as feature rich and they have a lot of really cool stuff we just don't have yet. That being said, we're also a different game. We're an MMO and our goals are to create a large scale world that gives you the incredible feeling of being a survivor in a zombie apocalypse.

We'll be starting out with roughly 64k square Kilometers, but we plan to grow it right away.. and over time create our virtual version of the US post apocalypse. Look for us to start adding land right away post launch.