I am using entropy since I first used Sabayon, I have to say that it is one of the easiest to use package manager out there and it does its job fine, however I think it would be a good idea to keep some older version of the same package, at least for the most important ones (like Gnome, KDE, XServer...).
For example, I upgraded recently to KDE4.6 and I noticed some kind of instability in the new integration with udisks, some strange behavior with Desktop icons and such.
Honestly, if I had the possibility to downgrade it to KDE4.5 and wait until the next version of KDE4.6, I would have done it.
It could help also for the stability aspect of Sabayon: if the new version does not work, you can always put the old version back
Maybe, in order to save bandwith, those packages could be in a separate server with limited bandwith, I don't know the details and technical aspects... But this is just an idea

