I have paid wildcard certs with them. Their site is weirdly designed and heavy on the self-service, but I have no complaints about them. I have revoked certs with them and everything has been reasonable.

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".

I currently use them and have no issues. I'm a validated customer and they take even the personal validations very seriously. They even check based on domain names, if you have financial in your domain name, be prepared to be questioned on why you are getting a free ssl certificate.