I used Ubuntu 7.04 but on my system. I did have some stabilty issues. Maybe it was me as this was the first Linux i ever used. There is an installation software called "Automatix" which really simplifies the process of getting multimedia eg dvd, mp3, divx, flash to work as these things are not installed out-of-the-box due to licensing restrictions. A few PC mags now have Linux sections, usually focusing on Ubuntu, and this I found very useful. I think Ubuntu will probably be the distro that has the greatest chance of getting mainstream success because the parent company has managed to get it installed on new systems from PC vendors eg Dell. Hardware support in Linux is always going to be behind Windows, but most stuff works, but I'd recommend nvidia-based systems rather than ATI, which now means Intel rather than AMD.
Since then I chanced on PCLinuxOS 07 and I found this to be very intuitive, faster and is probably better for Windows users migrating to Linux than Ubuntu. Personally I like the KDE desktop that is used by PCLinuxOS over Ubuntu (haven't treid kubuntu