Just got Ubuntu Mate (15.04) .img loaded and after some fine tuning. It runs great.
Just follow the instructions for downloading and installing the image (.img).
You don't need to do the build I just used the image.
https://ubuntu-mate.org/raspberry-pi/
after configuring I set up config.txt with this (yes I have heat sinks installed):
#uncomment to overclock the arm. 700 MHz is the default.
arm_freq=900
core_freq=450
sdram_freq=450
over_voltage=6
and I also found that using the MPEG-2 and VC-1 license keys by purchasing them from the Pi Store really helps with the screen redraw speed.
All in all, I'm impressed.
Total time to get it all running a couple of hours.
I have found a wealth of information on this forum, and just wanted to pass this along.