My co-worker broke her ankle earlier this winter so she's been taking a cab to work every morning. She's been a little dissatisfied with Green Cab throughout this entire ordeal. Though she calls far in advance to get a cab, they're frequently late, leaving her standing out in the cold on crutches (she has to get down the stairs in her apartment in plenty of time to catch the cab and she can't just go back inside if they're late). Yesterday, her cab was half an hour late. When she called to check in, she wasn't offered a discount or any kind of compensation for her lost time. They didn't even apologize for leaving her out there. She was late to work and had to take the extra time out of her lunch break. This is some of the worst customer service I've ever heard of.
I will also note that as a pedestrian, I've had more trouble with Green Cab disobeying crosswalk laws than any other cab company. Green Cab might be the cheapest, but there's a much better cab company in Madison who will always get my business first.