I bought this piece of awesomeness a couple months back and have been loving it ever since.
This should only be played for fighting games or arcade ports like Streets of Rage 2. The controller gives a fantastic feeling of using a real arcade pad to play. The joystick responds great and can take some punishment of hard turning and jerking during the heat of battle.
The buttons are also what makes this thing worth getting. The games respond just as well as they would to a standard Genesis controller.
Another great and overlooked feature is the speed control for your turbo buttons (X, Y, Z if the game doesn't use them). You can actually set the speed for them, it's quite impressive and helps out a lot on menu screens.
I totally recommend this if you have more than one fighting game in your Genesis library, because face it, it sucks not being able to execute Fatalities properly with those 3 buttons. Pick this up and be happy you did.