You should check out this article:
http://yoast.com/bad-wordpress-seo-plugins/ which has generated quite a bit of discussion. The author points out that one of the big features of All in One SEO pack is that it generates keyword Meta tags, which Matt Cutts has made quite clear "Google doesn't use the keywords meta tag in web search"
http://www.mattcutts.com/blog/keywor...in-web-search/
I personally think that good, lasting SEO is nothing more than writing engaging, intelligent, relevant content.