With the proper harnesses you can connect an aftermarket head unit. About the only thing that will work out of the box will be the antenna, the backup cam, and the various center console inputs.
For steering wheel controls you will need something like the Axxess box to interface between the Subaru language and the head unit. With the box you can regain most of the controls, but not all. You should be able to program the box to get good usable functions though.
As far as the mic goes, forget about it. The impedence is so strange that it just wont work with any of the modern head units. The same goes with the gps antenna. It will only work with an unit that has the same manufacturer of the hardware.
That being said, I love having my Pioneer head unit. It puts out more clean power, has more outputs, and makes expansion a breeze. And it sounds great!
For even more info, check out www.nasoic.com
or diyma.com. Don't forget ae64.com for all of the harnesses.