Been going to the ROM ever since i was a kid and definitely will be continuing to go in the future! The main exhibits seem to change/update VERY slowly (if at all), but what keeps me coming back is the special ones they bring in for a limited time (on exchanges from other museums). It's a huge place, so you can expect to spend all day just wandering around aimlessly. Staff are always very professional, polite, and helpful.
When i went they had the Mesopotamia exhibit on loan from a British museum. Lots of stuff about the middle east during the time of Hammurabi's laws and the empire of Babylon, etc. Nicely set up and very clean. Worth the extra price on top of regular admission.
If you plan on going more than once or twice a year the membership is worth it, going back after so long made me miss not having one anymore.