I have used it and I think they still have a ways to go and personally I don't think they will ever reach there end goal. The problem with the goal they have is they are competing in a market where Android and iOS are king and the way apps make money is selling power. Apps on Android or iOS make money through selling at a low price in large quantities or through advertisement that more active users that equates to more money made. The problem for Windows is it lacks the use of it's platform to obtain this goal to attract developers. Sure one could argue that Windows 10 here in the next several years will be the most widely used OS on PC, the problem is though that people still use a PC like a PC and smart devices like smart devices. I will be watching someone work in Word and then pull out there phone to post to Facebook despite having a web browser with a full keyboard and that smart device is never a Windows Phone.
As for the games, I will be even more amazed if they pull this off. If memory serves me right look at Gears or War that is offered in the store, it recommends hardware that blows the xbox one out of the water. The other problem with games for Windows is the "Walled Garden Effect". If you buy a game there Microsoft has complete control over how you play and obtain the game. Why would I do that when I can use Steam and play the games from various versions of Windows, Linux and Mac OS. I think some of the titles such as Gears of War, Halo and Forza will sell well but Microsoft has a goal to be a competitor to Steam and I am sure they will fail.
Either way the more people in the game the more competition which leads to innovation. All I am saying is I wouldn't get to invested in the Windows environment as Android and iOS are not going away and both Google and Apple offer some impressive PC options as well for those who want the "eco system".