My training partner and I (during the 767 Transition and Upgrade course) rented a car from Enterprise (across from the Atlas Training Center)...$118 per week divided by 2 times about 6 weeks is what we paid.
If you are in the 767 program, it is well worth the cost because the wait for the Marriott vans can cost you about 1 hour per day (to and from the Boeing facility)...and that 1 hour per day is not something you can afford to give up during the course (especially the 3.5 weeks or so until you take the FAA Oral).
I did not have a car rental during the 744 Transition a few years earlier, and that worked out ok...but then, several of my classmates did have wheels that I borrowed to get supplies, etc.
Having a car does allow you to get groceries from nearby stores and since you have a kitchen(ette) at the Residence Inn, you can save more time by cooking simple meals, salads, etc at a time when time management is critical.
One other thing...don't get tempted to take road trips to the Keys, or South Beach, unless you are well caught up in the program, or better, save those until you finish the training and have a blowout in Key West!!
Good luck.