For me it depends on the company. The large, "everyman" companies, the Comcasts and T-Mobiles of the world, have always had me dealing with a customer service representative on a script giving me friction about something that is completely within their reach, but they're made to be unhelpful until you cross some threshold. Start off polite, but the second they tell you something "isn't possible" you drop the polite tone, get stern, ask for supervisors, etc. That isn't to say swear and insult, and you should reset the politeness each time you get a new person.

In my experience anything even a little more "targeted" tends to have more reasonable customer service. I've had good customer service experiences from major sportswear brands, and we're not talking crazy high-end, boutique brands but major, publicly traded companies. That isn't to say they will just cater to your every whim, no questions asked, but if you have a legitimate grievance they will handle it without ruining your day.

Some companies care about reputation and customer satisfaction, and some just count every bean.