What do people think of SanityCheck.io?
We've been using it on our agency site for the past few months. It's been a great help in drilling down into areas we can improve and as such we have seen improvements across the board in our SERPS
Hands down, SanityCheck is the highest value SEO tool that we use (and pay for). It's simple to use, and highly effective at surfacing "low hanging fruit" on a regular basis. By addressing the issues and opportunities that SanityCheck brings up, we're able to steadily improve our SEO while juggling everything else.
But you can look the early stage of this product as an advantage - plenty of interesting directions to go in. The developer, Nick, seems to be interested in feedback, and is actively developing new functionality. He's interested in use cases, jobs to be done, not just adding another feature.