I have used all the free features that exist in SEOBOOK, there are a lot many sites offering similar online tools and the results are quite credible. I don't think you have to pay for the services that they charge as they are anyways available free of cost on other sites.
SEOmoz have tons of great tools, videos, documents etc for premium members so it's worth it. If you were to choose i'd say go SEOmoz and just use the free tools at SEOBook (Sorry Aaron) because the free ones alone are pretty good.