Any marketing tool you use likely requires some level of support. Just like web analytic systems and ad servers, an excellent (or poor) client support system will make or break an ESP's reputation and ability to retain customers.
It all starts with the people. Do you have a dedicated account manager that is your go-to person whenever there is a problem or question about the system? A great account manager can be the traffic cop when it comes to tech support questions, billing issues, and ideas for leveraging the capabilities of the tools. Someone who is not only a go-to resource when problems arise, but also offers proactive advice leading to email marketing success can often make up for other shortcomings your ESP may have.
Customer service and client services aren't just about tech support. They truly are opportunities to generate success stories and build relationships. Unfortunately, this mindset has gone by the wayside with many ESPs as they focus on centralizing support services in order to streamline tech support tickets and provide 24/7 technical support.
While tech support centralization is a good idea, it's not a replacement for the dedicated account manager. If you don't have a dedicated account manager, request one. If your ESP won't give you one, it's a telltale sign that you need to shop for a new ESP.