My wife and I have memberships at the AGO now. I've been three times since joining. I am trying to make it a point of going often in order to make the most of my membership purchase. (Though, I suppose supporting the gallery is worthwhile in and of itself.) The gallery is big enough it will probably take several trips to figure out where everything is, and see everything worth seeing.
The AGO renovation is amazing, and seems like what the ROM renovation should have aspired to. I like the Crystal, but once inside the museum it seems clear that it doesn't work that well. Gallery space is poorly utilized, and the interior just doesn't look that nice. There is all sorts of infrastructure jutting out here and there, which ruin the the interior elements of the design. The AGO on the other hand seems like a perfectly executed renovation. The exterior is certainly nice, but the interior is where things really shine. Everything seems neat and tidy and just right. The ROM's big success is moving its entrance to Bloor, and the new lobby which is quite stunning. Beyond that I think the renovation wasn't a success. It certainly doesn't seem to have captivated Torontonians the way the AGO renovation has.
The 18$ price tag is a bit high, but memberships are actually not too expensive, so if you plan on going several times a year they are highly recommended.