I have successfully installed Ubuntu pangolin 64 bit on a mac mini 2011 5,3 ( server version ). I have used the Ubuntu iso which I then converted to .dmg and dd'ed into an usb stick. I didn't want to keep OS X installed so I deleted the current partition and reformatted the drive as ext4 for / + a small swap partition.

Everything seems to work nicely, but I have now a question since I read lots of reviews/howto when you people installed rEfit on Mac OS X. Did you use rEfit to be able to have a dual boot or there are other advantages by doing that ( apart that you will be able to download updated firmware for your hardware in the future via Mac OS ) like BIOS emulation and the like ?