Expect to work 3 out of 4 weekends per month roughly. If you are a sharp swapper and user of open time that can be reduced if you can hack the minimum 2 local nights rest at base or are willing to back to back the 3 day African trips.
Blind lines thus far as someone 80-90% down the fleet list are pretty good. Preferences depend on how you use them - if you ask for lots of things you are unlikely to get much. If you make one or two, you are more likely to be in luck. Whenever I have needed a specific day or weekend off, solely putting that as your preferences or a call to preops have solved the problem.
Trip swapping with other guys on the fleet is what really allows you control of your life. In the months I've been on line, I've never flown more than one or two of the trips on my original line, and never once flown the roster I was originally given.
It's a great fleet, you'll love it.
Edit to say strange that my experience with preferences is so different, maybe Ive just been lucky thus far.