53
It s fully usable, every buttom you use... has an aplication but is steal in development and testing.
I am using in a site "in production" and it have more than 10 sites all working, more than 2 weeks on-line... and not problem happened.
Here is the email for MultiX quetions, sugestions, bug reports and fixes.
About you are going to be able to register in one site and use the same users in the others... the answer is yes.
Skalpa send me a "trick" to share tables easyly with out changing files or anything related, the trick is using MySQL 5.
If you are insterested
here is the post in the forum with the trick and a little fix.