Since I don't have experience with them I am looking for some level of assurance that they are a legitimate service. In my opinion it is difficult to gain that assurance just from their website.
That said, why does it matter if they're "credible"? Their certs are accepted by pretty much every browser, OS and library, and they have a long track record as a CA.
Regardless, as a business I have had business dealings with, let me assure you they are a "legitimate service".
In terms of "credibility", the issue comes down to how many browsers include their root cert by default. As far as I know, IE, Firefox, and Chrome include it meaning that it will be trusted by default.
The way they make money is selling other types of services such as wildcard and "green bar" certs. I think the folks running it want to see a wider use of SSL, and see providing free host-based certs as a good way to accomplish that goal. Bear mind, there zero cost to signing a cert ...