Creating Accommodation Multisite - Help needed with shared databases

Hi,

I am putting together an Australian Accomodation Web Network using drupal, and would like some advise for the best way to setup the shared databases.

Basically I there will be one main site (australian wide) that will list all the more specialised sites that i have (gold coast, brisbane, sydney etc). I want to have them all look and feel the same, and also want some shared features but not all.

I will be having a shared user table, but i was wondering if it is a good idea to share things like the "permissions" table or the "roles" or any similar tables like that.

I would also like to share blocks and things like that, but i think that will get a bit too complicated.

Any advice (and examples) are appreciated.