Squirrly SEO takes a much different approach from the other plugins in that it focuses on both tools and knowledge through slideshow tutorials. The company behind Squirrly SEO offers support assistance which is a real big deal. Squirrly SEO also helps you write better content by ranking content as you write and giving you ideas to improve it. The only negative aspect of this plugin is that it lacks more detailed settings that other plugins have. This may not be a big deal to people with smaller websites but if you're running a large website you need more control provided by other SEO plugins.
Please let us know if we have missed anything in the comments section.