As for more widely used packages, the initial update, after installing Sabayon on August 29 from an iso downloaded on August 3, updated KDE (or kdelibs) to 4.13.3.It also came with Chromium 36 by default and the necessary binary plugins from Chrome for playing flash in Chromium.
Mixing the systems is most likely not necessary for almost all users and would increase the difficulty of using Sabayon even more than as source based system only and increase the likelyhood of breakage, unless the recommended precautions followed.
Other less popular desktop environments are available by either first installing the minimal iso and installing the DE from withing the minimal installation or installing one of the other desktops from the ISOs and then installing the the desired desktop.
Besides the three major ones, LXDE is also available as a major category in the repositories.
On the user's installation end Entropy provides the Rigo graphical package manager and the Equo command line package management program. Someone who is used to apt may even like using Equo after learning its most used commands and options.
As stated above, almost everything most users would need is installed by default.
Sabayon seems to be a distribution that had been highly esteemed and popular in the past -- as evidence by the distribution version selection dropdown on the Opera browser download page listing Sabayon and by Vritualbox and VMware offering a customized install of Sabayon -- but has lost some of this esteem in recent years.