CentOS and Scientific Linux are both clones of RedHat Linux, and from what I've seen using Fedora, they have the same ease of use as any Linux distro. Many of the same apps are present in the repos of all these.
At present they all use the Gnome2 desktop as default (except Fedora, which has moved on to Gnome3). They have the same installer (Anaconda) and have the same access to proprietary drivers if one needs to go in that direction.
The main feature in the RedHat clones is stability. While Fedora is the bleeding edge of RedHat supported distros (Fedora is it's own community, but is supported by RedHat), the current RedHat clones are loosely based on what was Fedora 14. Fedora 16 is now in Alpha, so, RedHat is about a year behind as versions go, but, that makes for a stable OS.
While many of us like to run the most recent versions of apps, and help reporting bugs, when you offer a desktop or server to the enterprise, testing should be over, so, everything runs seamlessly. That's why there are distros like Debian stable and RedHat and CentOS.