So I finally picked up one of these - you may have seen the thread,
Sample Photos - M Lenses on Fuji X Cameras in this forum.
It's a quality accessory, and works as advertised. jairyhunter is right; it doesn't record the aperture info as the Fujifilm bodies lack the little sensor of the Ms. It does record the focal length and lets you dial in corrections (at least on the X-T1 and I believe X-E2); that's what the second button does. It brings up a focal length selection menu, and the info is recorded into the EXIF data. It's at the high end of adapter prices; in fact, only the Novoflex is more expensive - and doesn't offer the function button! Insane.
I'm currently testing a Kipon adapter; while it also lacks the function button, it allows close-focusing with a separate helicoid! Very cool. Also, being that it lacks the electronics, I can use it with THE lens I've been dying to use, which is "incompatible" with the Fujifilm adapter... The Voigtländer 35mm f/1.2 Nokton! Makes for a killer "50mm" lens.