Wow, reading these I'm glad I haven't had a truly horrible experience. I've never had anyone treating my rudely, but I have had people treat as if I know nothing about MAC (though they dont do it condescendingly). I suppose it's because I seem young, and they probably assume I have no money to buy anything with, or am new to make up (for some reason, it seems everytime I visit the MAC counter I'm not wearing any make up that day). But in general, they aren't too bad (however, I still had a really nice SA help me, even though I was buying a single Tendertone! She was really nice to boot).
I think the best customer service I've had is the MAC PRO store in Las Vegas (in the Forum Shops at Caesar's Palace). Probably because they're used to people coming in a myriad of shapes; old, young, dressed to the nines, dressed looking like you rolled out of bed! Every single MA there was as sweet as could be, so helpful, despite the fact that it was packed because of the Hello Kitty collection. It was multitasking at it's prime, and I was really impressed.