The Athletics Centre (or AC, as the UofT student knows), is the biggest athletics facility on the University of Toronto campus - including a massive Olympic sized pool, large indoor track, squash and tennis courts, many rooms for classes and a large change room.
If you're a UofT student, or considering buying a membership as staff or alumni, personally I prefer Hart House - in my opinion, it's a little less... intimidating.
The AC was built in a time when concrete everything seemed like a good idea - and thus the building is a labyrinth of concrete stairs, and generally uninspiring feelings. While they do have a great variety of classes, equipment, and other amenities, I am also often discouraged by the super fit phys-ed students running around the track like it's no big deal.
There are also many perks to the AC, which means that a lot of people (students and non-students alike), prefer it to the other campus facilities. There is a lot of new equipment, and it is constantly updated - as well since it is technically a "newer" building, many people prefer the showers and change facilities to other options.
Now, if I weren't paying only $50 a month for both this facility and Hart House, I might look for another gym - but if you're a student, alumni, or staff with a discount, it might be worth checking the AC out!