I started a new community with XenForo, link in my signature. Previously I have used, Vbulletin and MyBB.
Personally I think XenForo beats all the competition out there, yes it's missing things that people want(Blog [add-on available], CMS [Add-on also kinda available]) but the developers are concentrating on whats important and they listen to their customers. XenForo has great SEO, a forum with good content can get a lot of search result incoming users, my forums main traffic is via search results.
XF makes your users want to interact with your community. The alerts system, fast and quick posting/reply system and the general awesome speed of XenForo is a big plus point. Styling and customizing is painless, even a noob like me who never went into styling my own forum before gave it a hand in XF and got used to it easily.
The community here is great, support is great as well. In all XF is pretty awesome. My users have many times mentioned that they "love" the place (our site), I think the love is 50% due to forum software we use which makes the user like the community more 
