Hi there Yariome, as with most products, we couldn't really tell a product's maximum efficacy unless we see it for ourself, or better yet, experience it first hand. For a wider review on penis pumps, you may check out this article:
http://sexuality.about.com/od/sextoys/a/penis_pumps.htm or you could watch the video and handy guide here:
http://www.extremerestraints.com/penis-pumps_62/