Just like doctors they do make mistakes. I won't let one bad experience sour you on them. You also need to look at the number of systems they see in a day. It's a good idea to get in mid morning. Also make sure you've documented as much as you can to help them. Even we have a tough time with little or no information on what happened to diagnose a problem.
It's always a good idea to get a second opinion from another source. And we're even Free! ;-}