Hey,
I'm in my 3rd year at Macquarie and I'd definitely recommend getting the gym membership, but only if you're going to use it a couple of times a week, otherwise it's not worth it. I can't remember how much it costs but they do a deal in O-week of Semester 1 and 2 where you get a month free when you sign up with a friend. (If you don't know anyone, just hang around the sign in table and grab the first person who walks in by themself and pretend to be their friend. Believe me it works because the person you sign up with gets a free month too).
With the membership you get access to pretty much everything - the gym, the classes, the pool, the squash courts, etc. The gym are is pretty big, they have tons of treadmills so I've never had to wait for someone to get off. They did have a separate weights area for the girls and guys but the guys have pretty much taken over both and the free weights and weight machines are pretty popular so you might have a hard time getting use of them if you go in peak times (i.e. during the day on uni days).
The pool's really nice in the morning but is also busiest in the morning when everyone gets in before work and uni. The classes are awesome, my favourite part, and they have limited spaces so don't get too crowded, avoid going on a Saturday morning though - I swear they increase the class sizes and it's way too crowded. Also, you have to register for the class beforehand but you can't do it on over the phone and you can't do it more than 2 hours beforehand. Sometimes I register and then go to class and then come back but there have been a few times where the class has been full so then I just go and use the gym.
Hope that helped a bit!
Emma