I used it for a bit previously. But stopped using it.
It's a nice tool to see how you compare to other sites in the SERP. But at the end of the day, I felt like it was just about keyword stuffing and making your site look like everyone else.
The advice in the tool is good to some extent. But if you follow it, your site will look like all the other sites. I.e same amount of headlines, same amount of images, and same amount of words repeated in the article.
It can be handy, but not worth the money for me. If it was $10-20 a month, then I would consider it. But for 50 bucks? No way, Jose.
I think the reason why it got so popular is that they have an affiliate program and because the trendy thing in recent years is "content is king".