I've used SuperShuttle a few times without any issues** ...but not recently (several years).
** I don't know if their practice has changed, but SuperShuttle sets your rate based on the zip code where you are being picked up, and it has very little to do with rational metrics like the travel time or distance. They don't like my zip code, so even though its <15 minutes from the airport, they charge me almost double the rate that they would charge for Round Rock, Pflugerville, or Cedar Park--all of which are farther away from the airport than where I live. A cynical person might suspect that they were "redlining" or "profiling" areas with certain demographics.
Edit: I'm sure they would come up with some explanation about how their service patterns don't show much demand in my area so they have to charge more because the van won't be as fully utilized, yadda yadda yadda. It's still bullshit if I have to go to a friends house who lives FARTHER AWAY from the airport to get picked up so my rate will be $20 instead of $40.