I am using it as main OS. I liked it immediately when I was new to Linux because of the OOTB support for many components which normally require manual installation on other distros like proprietary drivers, codecs and so on.
It was a big DVD of 4GB with both GNOME and KDE3.5 (as default). With the switch to KDE4 Sabayon was splitted in two editions (GNOME and KDE), the effort of the devel team is the same for both. Just choose the DE you like.
Nowadays there are also other spins with LXDE, XFCE, e17, Fluxbox, but there is less effort on those, I don't like them that much.

Codecs (and hardware GPU acceleration through VDPAU and VAAPI) are configured by default and available since the first live boot, .

Can I have both KDE and gnome both installed?
So that I can switch between them, like in Mandriva, Fedora etc.

You know you can install both or all DE/WM with any Linux...

But KDE and Gnome of Sabayon are of two separate ISO files.

That one is the default de.choose one of them then install the other, that's it.