I am, but it hasn't been particularly great. There are some hardware issues, mostly surrounding audio and multiple monitors, that I haven't experienced when running Ubuntu on other machines.
Strange, Lucid works great without any problems on my Macbook Pro 6,2.
What model are you running?
Given the maturity of the software, there's no reason not just to run a virtualbox on Mac OS X with Ubuntu inside it. It's a hell of a lot easier than messing around with the partitions on your hard drive and worrying about hardware support. Unless you do very intensive real time work on the linux install, it's almost unnoticeable from a performance viewpoint.
The dual booting part should be painless, but as stanleydrew noted you may encounter problems with hardware support in Ubuntu.