I really enjoy the facilities at the JCC, and they offer a great rate for young adults (under 30). While the JCC certainly retains its religious identity way more than the YMCA has, make no mistake that people of all backgrounds and affiliations are welcome to work out and enjoy the programming here. There are two tiers of membership, the higher of which comes with towel service and a nicer locker room. I have always gone for the less expensive option and have had nothing but positive experiences. If you're driving in I suggest parking on Forbes ave closer toward Wightman street and walking. They do have a paid parking garage, but with a little bit of leg work you should be able to find ample on street parking even during peak hours.
Overall, the JCC is a very good fitness center, and it's a really nice feature of the neighborhood.