My bad experience with mechanics seems to stem around time. On two different occasions, with two different mechanics, I was told to leave my vehicle there all day. Well, that's my only transportation and I can't do that, so I told both of them I would wait in their lobbies... all day if necessary. Lo and behold, both jobs were done in under an hour. Now, I don't know if they were going to charge me for multiple hours, but I strongly suspected so.
A friend of mine told me the other day that the Toyota dealership he has to go to has a set labor price book they go by... a certain problem takes X number of hours to fix regardless how long it actually takes, and that's what you get charged for labor.