I found out about Cracker from my friend who got accepted by Penn State University. He spent 2 months on drilling the platform and ultimately scored 680. I personally spent a little more than that but finally achieved a higher score. It also seems to me that verbal tasks, especially SC questions, are a bit harder than those in OG.
I would recommend Cracker to those who have at least mere understanding of what GMAT is. The platform is full of explanations, and what I like most is the list of terms in the toolbar. Whenever you get stuck and cannot remember a certain formula or property, you can simply go to the list of terms and find the needed concept.
Cracker is a comprehensive platform with tons of questions and explanations. Progress analysis is a very helpful option that lets you trace your progress and work on your weaknesses. I bought a supreme pack and discussed GMAT questions with tutors almost every day. Eventually I hit 740!