I’ve had an X for the last year, and while I love the phone, there are a few issues with gaming.
While a lot more apps support the aspect ratio of the X, there are still some games that have never been updated for it. No Playdek games have been updated for the X. Coming from a 6+, it was frustrating at first to have the effective screen size for games smaller than what I had before, when I had expected it to remain the same. Fortunately Czech Games and Digidiced eventually updated all of their games.
The other significant issue with gaming on the X is gesture navigation to the home screen. Since minimizing an app is done by swiping up from the bottom of the screen, games that involve sliding cards from the bottom of the screen (Ascension and Through the Ages particularly in my case), can be annoying at some point. Ascension in particular, if you accidentally minimize the screen when playing your first card will result in having to return to the app and watching your opponent’s last turn all over again. I’m not really sure what the solution is here (maybe having the option to change gestures?), but I don’t believe there is a reasonable one a year later.