A car is nice but definately not necessary...although with two classes going at any given time, the hotel shuttle is running non-stop these days. A car will let you go to more places when you want, and there is a Golds Gym not too far away than you can get a 4 week membership at for 35 bucks. A parking pass for ATW costs you $20 bucks, but you get it back upon returning the pass.
You'll get your crew ID at the end of your second week, and a few people try to get home, but plan smart and HAVE A BACKUP PLAN...there are only so many free seats into ATW each day. Using GRB and giving somebody 20 bucks to come get you is probably cheaper than renting a car at ORD or MKE should you not make it into ATW.
The only potential downside of a car is that depending on sim schedules, you might be shipped out to CPT immediately following the end of systems. They did say they would provide parking in MKE if you needed it, and obviously its easier to get to MKE than ATW. You also have the option of not using their travel to sim and "finding your own way", with the warning to make sure you show up on time for your events.
ATW's a pretty decent place when it isnt snowing and the wind isn't 25kts or more in your face...enjoy!