I put Ubuntu 8.04.1 LTS on every machine whose owner can be convinced to let me stick my grubby lil hands in it. Both my and my client's lives have been made much happier, fuller and more satisfying because of it.
These clients of mine, however, would never, ever install Ubuntu or any other distro on their own. They see the machine and it's OS as an inexorably intertwined unit; not something that they can control or modify to suit their needs... Microsoft's success has been due to Windows being *pre-loaded*, and I think that as more and more manufacturers are willing to sell Ubuntu or openSUSE-based desktop systems, and perhaps now that the lawsuit between Pystar (http://store.psystar.com/) and Apple is over (I think...), we may see a decline in the Windows monopoly on the desktop as more and more companies start selling Linux and Hackintosh systems.