I've taken SuperShuttle to the airport for early morning flights twice before and had a solid experience. This time, I was getting in late at night and didn't feel like transferring buses, so I booked a SuperShuttle. Then I was able to get on an earlier flight (and moved my shuttle reservation after being told I couldn't cancel), and I ended up getting to the airport around 6:30pm. I show up at the ticket counter to check in and there are about 6-7 people in the waiting area. I overhear one of them saying "she said it could be another hour." That's when I start to worry.
Sure enough, even though a shuttle comes about 30 minutes later, my number is not called (despite the fact that we're now 20 minutes past my reservation time). I end up waiting about 75 minutes for a shuttle. FYI, they call you up in the opposite order for drop off. I live about 20-30 minutes from the airport, and an hour and a half later, I was home. I walked in my door at 9:15pm, more than two and a half hours after checking in for my "reserved" shuttle.
I will NEVER take SuperShuttle from the airport again...but for going to the airport, it's a decent low-cost option for when the buses aren't running.