They have a fairly extensive API, I used it in a small business, my main problem with them was they kind of just sat on their laurels and didn't keep up with other companies in the space. Once you get a few thousand contacts (who in our case were very casual "want to keep up but don't want to buy a lot" low-dollar-value types), you're paying a large sum for a lot less than competitors give you with a full marketing automation suite. We've since frozen the account.