What do you guys think is the best price/feature ratio for that kind of service? I can't find any detailed feature listing on the website, but it seems you can't offer much for ~$1 a month. Do you think 15 minute interval is enough? I assume the checks are made from one network location only (can't find details on the about page), would anyone be confident of the results with such setup? Is there any re-check in case…
15 minute interval: I had to make sure the site was scalable in the beginning. My early testing data tells me that, for my target audience (as you say, probably not larger businesses at this point), 15 minutes is a good interval—it's better than most free services, but not so frequent as to incur extra expense on my end for bandwidth and database growth.
One location only: This is a compromise I have made for now—I have built the service in a way that I'll be able to add another node or two for the back-end checks once I get enough users. But for now, if the main server is down (hasn't happened yet, in four months of testing, but it will...), I only have a small bit of contingency, and don't have a hot backup.
Re-check for failure: Yes; if Server Check.in notices a site is down on more than 50% of node checks (once I have more than one node), it will check the site once more after a minute before sending a down notice.
API access: I don't have it yet, but I'm going to be building it (everything I've done on the site already has API data structures in place—I just don't have any of the interface or any kind of authentication system in place). I'm working on a mobile app that will be free for users, and will need the API up and running before I can launch it.