I think it's a good idea for those that did reviews to share some of their experience. I read some of the reviews submitted and some of them were very comprehensive. It was clear the reviewer spent considerable time and effort, and the contribution is appreciated. I wonder, however, if this may be intimidating to those considering submitting a review. Some may not feel they have the time to commit to submitting a review.
Outside of taking the time to read the book, the writing of a review probably shouldn't take too long, especially since you have nicely broken down the different areas to be reviewed (Relevance, Clarity, Consistency etc). If a potential reviewer knows a little more about the time commitments, they may be more willing to give it a shot.
FWIW, I think that there is momentum building towards this project, even if at times it seems a little slow. OER has been a constant source of conversation at most system meetings I have attended in the past year. This was not the case the year prior. As greater awareness is achieved, I think you will find greater levels of participation and adoption.
Cheers,Mike