If I could do it all over again, I would go for the paid critiques, but I feel I have gotten a tremendous amount of value out of doing the exercises even without them -- you can look at my completed lessons in my profile and judge for yourself -- and I have also been lucky to receive some valuable feedback from my peers who have taken the time to look over my work. It has been extremely useful to me to look at the completed work of other students and the official critiques they have received before I am done with that particular lesson. This will prime you to recognize certain pitfalls before you fall into them (or will teach you how to pull yourself out of them), and can give you a leg up on the lesson if you can implement some of the critiques and suggestions offered by the evaluators. I think this question ultimately comes down to two things: 1, whether you can hold yourself accountable to do the lessons as they are written without the social or monetary pressure to do so (from a cursory glance at your completed lessons, I would certainly say you can); and 2, whether you are the sort of introspective person who can look at the work of other students, then look at the critiques they have received, and then evaluate your work in that context in order to see if elements of those critiques might be applicable to your drawings. This is not a matter of judging whether your work is as good as the work of other students, but instead a more objective sort of evaluation where you esteem how your understanding of the tasks of that lesson is lining up with the work of other students.