Even when I have a meltdown with customer service (seems to happen with my ISP with regularity), I always say: "I appreciate your help". Though, if I have to keep calling about the same unresolved issue, I politely ask to speak to the team lead and then let it rip without mercy. On several occasions I have managed to escalate myself to a senior executive (a CEO once) who should NEVER get complaints directly from a customer. Problems get resolved quickly once you get to the V and C-level.