I treat them as I would any other service provider and leave my judgement until they do something wrong, or something right, and then treat them accordingly. My first response will always to be polite, then firm, but after an hour of them not recognising/rectifying the problem I will start to get a bit angry at them, and will sometimes walk out (after paying under duress) and calm down and write a initial letter to their service manager & DP pointing out the issues in a brief, non-ranting manner and the expectations I have.
I'm yet to find a dealer that deserves a pat on the back. I've not had one visit in over 10 years where everything has been done correctly, or where they've not tried to charge for something they've not done or for something I've not been asked about.
It might only be something as simple as not resetting the OBC, washing the car against my wishes, not supplying me with inspection reports (tread depth, pad/disc thickness, valve clearance measurements) but they can't expect a 10/10 or 5-star rating on my feedback form if they're not doing a proper job.
I've had such lackadaisical service over 10 years that I just expect it, and if they've got a couple of small things wrong I'll just smile and ask them to rectify it.
Sometimes the service agent/manager can't seem to comprehend why I'm concerned that they've missed something (such as not putting coolant in, or leaving the oil filler cap off, or not tightening the wheel bolts). But when you realise that the work has been carried out by an experienced technician, or an apprentice, and then signed off by a supervisor, you wonder whether they're doing the checks at all. That's why I started UV marking some service items (the parts themselves, or where that wasn't feasible I'd mark the bolts/gaskets).
My ///M specialist does get beer/wine/cognac now and then though - especially when he'd spend an hour fixing a problem that BMW wanted £1000 for. So it only goes to BMW for warranty & servicing - general maintenance (brakes, interim oil changes, interim brake fluid changes, etc.) is done by my specialist in deepest, darkest Essex. As he's the boss I know where the buck stops.