For $50, you get a tiny monochrome screen, a D-pad and TWO WHOLE BUTTONS, and a non-rechargeable 9 hour battery.
A coin cell battery? This is 2014, and it doesn't have a rechargeable battery? He did mention you can get rechargeable coin cells, but they only last an hour. So with moderate use, you're gonna be spending a few bucks a week on coin cell batteries, not to mention having to carry around spares with you.
Yes, it's nice and thin, but nobody is gonna stick this in their wallet unless he invents a flexible PCB.
Or, for $30, you can get a brand new LG smartphone from Walmart, running Android 4.4, with a 3.5" color touchscreen, a dual core 1.2ghz processor, camera, wifi, bluetooth, GPS, accelerometers, speakers and a microphone, several gigs of storage space, and a RECHARGEABLE battery that will last 10 hours heavily used and over a week in standby. I guess the only downside is that touchscreens aren't as good as D-pads and physical buttons for games, but you could always buy a USB controller and get a lot more buttons. And while it IS about the size of a credit card, but half an inch thick. And instead of shelling out a few bucks a week on coin cell batteries, you could spend that money on a straight talk cellular plan and get UNLIMITED INTERNET ACCESS!