Mainly because I run it on my Community site (
www.nzboards.com) and the results I saw on that site were so good (PR3 to PR5 for main page, PR on individual threads etc) that I decided to also run it here.
I used both DeanC's vB SEO URL rewrite and vBRanking (which was stolen code from the early non-encoded beta days of
vBSEO), and neither have the HUGE range of features and options of
vBSEO.
For a free alternative, Dean's is the best that I know of, but it doesn't come close to
vBSEO.