I think schools that have an additional fee for the use of on-campus gyms are the execption rather than the rule. I think most places have an "activity fee" or "recreation fee" that covers gym use. At IU, you pay mandatory fees that cover gym usage, however there is a living community (a few floors with in a dorm) with an additional gym that you can use, which carries a higher fee than other floors in the same dorm (like $100/semester).
As for quality, I think it's typically going to be below that of a private facility. This is just because you have more people working out than at a private gym (where have you seen a Bally's with a line of people to get in?), and more people who don't know what they're doing. So unless the gym is brand new, or has brand new equipment or was recently renovated, I doubt you'll find that it's up to the standards of a private gym.
Also, gyms are typically of two types: older, "fieldhouse" type facilities that have been at least kept in decent condition, and newer gyms that more closely resemble large health clubs.