The Gripper is a simple rubber thing that just slides on over the keypad. It's been more or less hand made, that much is easy to tell, but it feels durable and its size plus flexibility means it'd be comfortable enough to stick in a pocket or can be tossed into a bag without breaking. Our initial fears that the buttons would have horrible feel turned out to be unwarranted, as they actually have a decent tactility to them. It's nothing like the feel of a standard gamepad, but it's not necessarily bad.
We played a number of games for a number of hours and about the only complaint we have is that the Gripper has a tendency to slide left-to-right a bit when you're frantically tapping in games like Street Fighter 2. But, that could be seen as a good thing, as the unit comes with a rather scary warning that pushing too hard could potentially damage your phone's keyboard. Perhaps we're being a bit optimistic, but that sliding does let you know when you're getting a bit too forceful. Regardless, it rarely slid so much as to cause us to hit the wrong buttons.
After those hours of play we never noticed any change in feel on the keys being pressed, nor any marks on them or any other sign of damage, so we're reasonably sure you can load up those emulators to your heart's desire and get your game on without fear of permanent damage -- if you can rein in that super-human finger strength you've been developing all these years.
For $15 the Game Gripper turns a Droid (or Milestone) into a far more capable gaming machine and, while we do wish there were some way to add shoulder buttons to the thing, we were actually able to map the camera and volume up buttons to be R and L, respectively, giving a reasonable facsimile for an SNES controller. The feel isn't perfect, but we're satisfied, and after a few days with one we certainly don't want to game without it.
Oh, and if you happen to be playing on some other device, you'll be happy to know that there's a Gripper coming for the Devour, Backflip, and the N900 too.