Best support in the biz. We switched over because they're well-known for their client support.
I don't know that I can think of anything...
We were in the process of renegotiating our contract with the ESP that we were using when we started talking to Adestra. They met us in a fair place on price but more, when they demonstrated their product, they said something key, that they had live support available via email with quick turn arounds, phone or chat. Our previous ESP was supported through wiki articles. Huge value add.
Since signing up with Adestra, they've been a great group to work with. Our account manager has been readily available to offer tips and assistance, the online chat function is a quick way to get fast responses and support emails have been responded to within two hours of sending them with solutions.
I would highly recommend Adestra for their product without the added service but when you add that, it's a win!