Are there downsides? A tiny few. I'm not crazy about the keyboard layout or the decision to use multiple rows of special keys right above the standard Function keys, because it means I tend to whack them when I'm reaching for a hotbar instead. I'm also not thrilled with the location of the "Delete" key.
If you're in the market for a gaming laptop the ASUS G751 is the new gold standard for everyone else to beat. And we want to give credit where credit is due -- NVIDIA's GTX 980M is easily the best mobile GPU architecture we've seen to date. It's far better than Kepler-based hardware. AMD's Apple iMac-only R9 M295X would undoubtedly make a better fight of things than the R9 M290X did in our Alienware 17-inch review, but based on relative performance on the desktop the GTX 980M would still most likely win that fight in mobile.
For all the credit NVIDIA deserves for making a great mobile chip, the crown star still goes to ASUS for integrating that chip into a fabulous mobile system. If you're in the market for a mobile gaming rig in this class, by all means, give this laptop some serious consideration.