The ease of use and their documentation is superb. Allowing me to hook up to the service very easily. They also have a wide offering of services that I haven't taken full advantage of yet, but nice to see they have a good ecosystem.

It has an API that's easy to use and implement, and it just works. I like the notifications that are sent so you can keep an eye on usage amount and credits remaining. Support (the couple of times I've needed it) has been great.

Ease of use & Price. Great system that integrates well.