The core functionality has been implemented for Xbox 360 controllers, and it feels great! The configurations I have are:
You'll be able to play the game with a friend if you only have one controller. For solo play, you can play it with either Keyboard + Mouse or a controller.
So far, I only plan to support Xbox 360/One controllers and Steam controllers.
For game programmers out there, I'd like to share a word of warning to those creating a game in Unity 4 with multiple controllers: I've found that for Xbox 360 controllers on a Windows platform, using Axis 9 and 10 to register Left and Right triggers will not work if you have multiple controllers plugged in. It will require ALL controllers to press down their triggers to register a trigger. Use Axis 3 instead. I'm using Unity 4.7; I don't know if this bug has been fixed in Unity 5.
Input.GetJoystickNames() also returns blank controllers sometimes, so make sure you check for those as well! These two bugs have caused some unnecessary trouble. Unity is still an amazing engine though!