* Customer service is maybe worse than Comcast. It took me a month initially to figure out how to give them money - the sales rep just wouldn't call me back, no matter what I tried.
* They've got weird data holes. Sometimes one of their services will recognize an address and return an address key - while another one won't recognize that address key at all. In addition, about 5-10% of the time, the address will be a real, real-world address, and MD won't recognize it.
* Their Property service won't recognize addresses - only address keys. Which means that you have to do a separate lookup on a different service to get the address key, and only then do a lookup on their Property service. That's kinda nutty - it makes it more complicated to pick the right mix of services you need, and also makes it slower to do your lookups, since you have to do two of them.