I've had an R50X for a little over a month. I have about 370 miles on it so far (time for first servicing!).
TGB's promotional literature claims it will get 60 mpg. My last two fill-ups have each come in at 57 mpg. I'm about 230 lbs and my kids are frequent passengers so that is undoubtedly dampening things a bit. All in all, the claim seems pretty accurate.
By the way, this thing really sips oil. I was expecting to have to keep up with crank case levels more but I've only had to put in an ounce or two so far.
Rod