If you are console-centric as I am, then a controller just feels right for gaming. I own one computer and I have never been much of a PC gamer. I spent my youth with a console and my brain is now wired to "video games require controller". Owning a PS3 I decided that I would give the DualShock 3 a go with my Mac and am incredibly happy I did. My aging (and loud) PS3 has taken a backseat to my Mac for my gaming needs since I plugged this puppy in. From all the games available online for Mac (Steam, GOG, MacGameStore, HumbleStore...) to all my old favorites by way of emulators, this thing works wonderfully well. I even bought a white one to match my Mac!
The benefit of using a PS3 controller (and PS4 now) over an Xbox 360 controller is that the drivers are Mac compatible and it is a simple plug and play operation. Plug the controller in using the USB cord it comes with and press the PS button to turn it on. Connected! Also, Bluetooth works whereas the wireless setup of a 360 controller requires a dongle to receive the radio waves. A quick search on Steam came up with over 800 games that offer full controller support and I know of some (coughcough Arkham City) that were not listed but definitely are controller supported.
The downside of using a PS controller is that most games are designed around the Xbox button system. That uses a button layout of A, B, X, Y where Playstation uses shapes: Square, Triangle, Circle and X. So you have to adapt a little to onscreen prompts and rely on muscle memory (the buttons are in the same spot, just different labeling). Also, Bluetooth pairing was a little confusing. The lights on the controller flash constantly while plugged in and when unplugged but connected via Bluetooth. Because I was familiar with the Playstation 3, I thought the controller was not working when the lights flashed. Incorrect! Flashing lights when connected to Mac = good! Big Picture by Steam is a great way to test if the controller is connected and working properly without having to load and quit out of a game, by the way ;).
Not every game I have tried to play has worked with the controller. The Wither 2 is a perfect example of this and if you are really determined to get the PS controller to work there are several controller mapping programs out there. I have used Tattleboogie and Controller Mate with some success. You can go so far as to replace your mouse with a controller using programs like these.
So! What are you waiting for? Grab a controller off of craigslist or dust one off the shelf, load up your favorite brawler or emulator and have some fun!