The official strategy guide goes on sale June 19th. The significance of this is huge, because these guys have refused to post any indication of when the game is actually coming out, including banning everyone from taking preorders. The strat guide coming out in a few weeks is a huge indicator that the game will be coming out very soon. Potentially even as early as the end of June/beginning of July.
I honestly think this game is very over priced atm. For one its on the PC which the Xbox version will cannibalize this one, as well it has a stupid subscription plan which offers advanced features that should be there anyways like customer service.
The fact it is on PC makes me have confidence in this stock. It's a spiritual followup to Diablo (even though it's in a very different setting), and we know PC gamers can be very loyal to certain franchises and spiritual successors.
It's the biggest platform in the world and while not every PC owner will use it for gaming, the PC owners that do play games tend to be fairly fanatical. As long as it has a single-player component that's fun enough to warrant a purchase based on that alone this game could hit a million in the end. PC doesn't see too many anticipated releases these days, so you bet some of the avid PC gamers gobble up every release that's even slightly worth their while.
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Source: http://au.pc.ign.com/objects/737/737992.html
(I tried to suggest a date on the Wiki Date but I'm getting errors.)