Regarding your unstable vs stable question, Sabayon Linux is customised Gentoo Linux from the Gentoo testing/unstable branch, and the vast majority of the ebuilds available in SL are thus those in the testing/unstable Gentoo Portage tree. There is a Sabayon overlay containing relatively few ebuilds when compared to the main Portage tree, and the ebuilds in that are thus all in the testing/unstable branch.
If the latest version of an ebuild in the testing branch of Gentoo is marked as unstable then the same applies in SL because it's the same ebuild as in Gentoo. If the latest version of an ebuild in the testing branch of Gentoo is marked as stable then the same applies in SL because it's the same ebuild as in Gentoo. The SL devs do pass fixes upstream.
There used to be a stable branch edition of Sabayon Linux (it was called "Professional Edition", and, before that, "Business Edition") but it is no longer available or supported.
As far as SL's binary package manager is concerned, the binary packages in the Entropy repository are built from the above-mentioned ebuilds and thus all have exactly the same status as the ebuild counterpart as far as the branch is concerned. Entropy just provides pre-compiled Gentoo (and Sabayon overlay) packages.
I think Entropy enables a more stable installation than a source-based distro because the Entropy packages are pre-built. It's certainly more difficult (but not impossible) to break an installation in which you use only Entropy. Actually it's quite useful to have the choice of using Portage and Entropy: I tend to use Portage on my main laptop (but have found Entropy useful occasionally to rescue things if Portage breaks something) but on another laptop used by one of my family stick to Entropy for ease of use and greater stability.
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Regarding your question about the pace of the distro, turnover of devs and the activity of the community, I can only answer as someone who is not involved as a developer, only a user of the distro.
You can read about the development team from the link on the Home Page (Communicate > Organisation) but here it is to save you the trouble: http://www.sabayonlinux.org/pages/show/id/62
. As you can see, it's a small team, albeit very dedicated. These guys have been around quite a while.
Regarding the pace of the distro, it has been very active over the more than two years that I have been using SL. You can gauge the rate of development by looking in the Press Releases
'shed' of the SL Forum, and also by looking at the announcements and road map in the SL developer's blog (a.k.a. 'Planet') - see the link at the top of this page. I think the pace has been exceptionally good, and the quality of the whole package has improved with every release, and each release has become slicker and better than the last. I'm having trouble with fglrx (my GPU is no longer supported by AMD), KDE 4.2, BlueZ etc. at the moment but, as I'm sure you know, they are not distro-specific issues. Bloody Linux (and AMD)!
The SL community is much smaller than the Gentoo community. That said, it seems to be quite loyal. There are a core of users who are clearly very devoted to the distro and you see them appear regularly in the SL Forum and are generally quick to help and some are very knowledgeable. As you know, Gentoo is not easy for newcomers to Linux (or from other distros, come to that) and I have seen many users come and go in the Forum. The introduction of binary packages and the home-grown binary package manager Entropy -- the brainchild of lxnay
, the father and principal developer of SL -- has attracted new users and made package management a lot easier and faster for newcomers to Linux and for people used to binary distros.
The SL forum can be a little quiet at times, but you can judge that for yourself by browsing the 'sheds' and the Active Topics search. Actually, a quieter forum can sometimes be an advantage: threads don't disappear off the bottom of the forum page faster than you can say "Ubuntu Forums", for example.
By the way, did you know that the Cuban government's official distro, called Nova, is based on Gentoo and uses SL's Entropy as its package manager? Cuba crafts extra-communist Linux distro