No doubt there are great mechanics including a number that post on this site. Then there are the rest of them, some dealers and some not. I also had to correct major work by a subpar shop and I agree, it is hard to believe how frustrating this can be. I am still diagnosing problems they introduced looking for more vacuum leaks they created by using old o-rings and seals rather than new ones in the fuel system work. Of course you should not bring new work to that dealership and in general always be present, watching the car, when having a shop correct faulty work. If that bothers them to bad, If they wonder if you dont trust them the answer is not necessarily.