Mack has been recommended as a good source for a rental car but our recent experience was anything but pleasant. We opted for the full insurance package, but were informed that a $1000 charge would be blocked on our credit card until we returned the car. We were also informed that any damage needed to be accompanied by a police report in order for the insurance to kick in and cover the cost of repair. The next day we tried to turn in the car early (three days early in fact) and were told that we could do so, but still had to pay the full rate for all four days.
Mack's website mentions nothing of this policy, and in fact implies that early returns are acceptable. Given their ancient business practices and lack of cooperation on helping with simple travel arrangements such as we encountered, I'd highly recommend against Mack and encourage drivers to stick to major companies like Hertz.