You really should go and visit the gyms during peak hours to see what they are like. When I was shopping around for a gym a few years ago I became less concerned with price and more on equipment and amenities as well as service and the amount of "meat heads" that were hanging around the gym.
I ended up at Xsport on 8th and State. They had 2 hrs of free parking and I got an extra 6 months free when i signed up. I dont think it was the cheapest dollar wise but when you factor everything in....(some places make you bring your own towels)... it was the best choice for me.
After moving to the South Loop, I joined FFC on State and Roosevelt. For $64.95/month, you get a super clean gym, towel service, weights that are always re-racked, and the busiest area seems to be the cardio. I usually go there around 6:00-6:30 at night and you can always find an open treadmill, bench, etc. The $64.95 package also give you access to a few other FFC locations including the West Loop FFC which has a pool.
Interchange Medical Wellness Center (above Trader Joe's) has a different pricing model which works really well for me. It's a pay per service gym where you can do group classes or private/semiprivate training and pilates. So no commitment necessary and plenty of options.
I just moved to the Sloop one month ago and went through all the gyms to compare services and prices. For me, the decision was an economical one. In short, the best two offers came from XSport and FFC. FFC had a large up-front enrollment fee plus about $65/month. XSport was running a deal where, if you pre-pay one whole year, they would waive the enrollment fee PLUS give you an additional twelve months for free. In total, it is about $400 up front for 24 months of membership. Needless to say, this is the one I chose. The promotions change monthly, however, so you would need to check in and see what they are running currently.
I have lived in the South Loop for 7 years and have had experience with three different gyms. Initially, I signed a contract with X-Sport for $33 a month. X-Sport is a nice gym with all of the amenities one could ask for. I terminated my contract after my locker was broken into and all of my items including clothes, wallet, keys, phone, etc. were stolen. Reluctantly, I joined LA Fitness, which was a decent gym for a price of $30. I left this gym because I didn't not feel comfortable with some of the other patrons who worked out at the club. The final straw was when I was working out and noticed that the guy across from me had a very ominous statement tattooed across his forehead.
After my experience with the aforementioned clubs I realized that anyone can afford a $30 a month gym membership. If I wanted to work out in a gym that was clean, modern, and had a quality clientele, I was going to have to pay for it. That's when I joined FFC in the West Loop. This club has a very high clientele, all the bells and whistles, and even an outside pool and bar area for the summer. Yes, you do pay a little more ($65/month with a big enrollment fee) but that keeps the thugs who typically frequent lesser gyms out of FFC. Fitness Formula also has a lesser quality gym in the South Loop, however, with a membership you can use several of their locations. If you can afford it, this is the gym to go with.