I've been using Ubuntu 9.04 as my main desktop operating system at work now for a few months. I am very pleased with the switch from openSuSE 11.1 to Ubuntu. Everything so far works in a predictable way and I think Ubuntu have done a better job of the Gnome desktop than openSuSE.
I use the machine mainly for PHP web development, UNIX application development and general Java development for embedded systems and server-y things, OpenOffice document publishing and other general office tasks. Ubuntu is well suited and easy to drive for these purposes.