The two things you have to consider are:
1) Park hours: The longer the park hours, the more FPs are available, so the more time you have before they run out. Short park hours (like the 10am-8pm we had during most of the weekdays of our Oct 30-Nov 13 stay) mean those passes can run out quickly and you should adjust your expectations for either how early you need to arrive to line up or whether to just ditch the FP idea altogether.
2) How badly do you need to go on the ride? We really wanted on our first day, and used Mouse Expedition to get us FPs while we did MM and then another hour in DL before heading to DCA. Awesome. That was the only day we used FPs. Otherwise, we would be in Carsland at least once a day and just checked out the standby line. We would go if it was 30 minutes or less, or, once, we went right at park closing, when they still let you in the line and a technical 8pm close actually became an 8:45pm RSR ride time. We saw the breakdowns or heard about them while in other lines, but were never affected by them. Once we got that first ride done, it just became something to do if relatively convenient.
Same with the other Carsland rides (and TSMM). We found the CL lines, other than RSR in the morning (read: always loooong), to fluctuate a lot. You never knew if Mater's or Luigi's would be a 5-minute wait or 30+, except in the final hour when they were short lines. One hour it could be a long wait, and then we'd see "Aladdin" or "Disney Jr." or visit the Animation Academy or Sorcerer's Workshop (highly underrated) and then come out and have a really short wait. So, decide if you're flexible and willing to check out wait times before committing to riding that day or if it is a Must. That will help figure out whether The Line for the Line at the FP machines is worthwhile.