I sometimes use ubuntu on my PB. If you REALLY like to customise your computer it's very useful. The main thing I use it for is multiple desktops.
If you're considering installing it on your Mac, do what I did and make a live CD of all the distros your considering and try them out before you take the plunge and install them. On the Debian website there's a few pages about installing Debian on a PB and getting everything to work, which would also be useful for people installing Linux on other Macs.
It is possible that you'll have to backup and do a clean install of OSX to get the partitions you want, but not always.