Well from what I read it's getting better with each update. The plugin the IPB supports internally is IP.SEO and the URLS are rewritten nicely and creates a sitemap.
I have been having problems with creating the sitemap along with a few others. http://community.invisionpower.com/topic/361500-problems-generating-sitemapxml/
From what I have noticed is IPB SEO is generally average .. not good like Xenforo and not worse either... but it does has issues.