For everyone else, Red Hat/Centos is primarily for setting up servers. Red Hat is getting closer to the better package management of a Ubuntu/Debian (allowing one-click install or update of everything), but it's not really geared toward easy desktop use like Ubuntu/Debian. There is a server version of Ubuntu or Debian too depending on your purposes. The deal with Red Hat, being one of the few commercial Linux distributions, is that if you buy proprietary sever software (from Oracle/BEA, IBM, etc), they will only certify their software on commercial varieties of Linux (Red Hat, SuSE) rather than the free ones (Ubuntu, Debian, etc), because proprietary companies don't have controlling influence over free distributions.
For desktop purposes the nice thing about Ubuntu is that it comes in 4 different flavors with pre-configured window managers: Standard Ubuntu (Gnome Desktop), KUbuntu (KDE Desktop), XUbuntu (XFCE Desktop), and even an Edubuntu (also Gnome, but for kids/education). You can of course customize any one of them, but the purpose of those (and any Linux distribution) is pre-packaging of software they think you might use the most.