As far as I know, various Unix versions, as well as Win7, will run just fine. No artificial performance penalties, and Apple is currently doing precisely NOTHING to disallow installation of alternate OSes. This has been the case for years.
Except getting macbook to boot anything else than OSX or Windows requires bunch of stuff that I can only describe as 'hacks'. At least it was like so couple years ago (http://mjg59.dreamwidth.org/12037.html) and I haven't heard that things would be different today.
Running Linux installed by this method on a (2008) Mac right now. Have done the same on recent (2012, 2013) models, too.
Macs have been using EFI boot for a few years, so the problem most have probably faced is trying to boot in BIOS emulation mode (which I've found to be iffy on pretty much every EFI motherboard I've come across).
I did, a few months ago. Booted both Fedora 19 and 20 Beta on a 2012 13" rMBP. Worked decently well, especially with how the version of Gnome included in F20 knows how to properly deal with high DPI displays.
I've had good luck booting Fedora (at least 19 and 20, and I think 18) on Macs.
Other distros don't seem to work with Apple's EFI implementation. To boot other distros, I've had good luck with the refind[1] bootloader. Granted, I have only been booting live USBs, not installing on the internal drive.
