I have used Epsilon and I have evaluated Silverpop, e-Dialogue and Yesmail. From my analysis Salesforce MC is far more superior in flexibility, scalability and freedom to custom built initiatives. The AMPscripting is one of the driving forces in allowing the previous mentioned abilities. However, I can see from a marketers perspective where they simply want to send emails and segment easily. But given that effective email marketing requires a lot of different components and there is no simple way of doing it and I found that most companies that offer the out of the box solutions do a good job but there is no one size fits all and if you are trying to be really good at it which includes integrating your business such as registration, stats, CMS, API, etc. This is a much better solution.