There aren't many choices left. vBulletin has imploded, becoming a bloated and unusable mess with (last time I checked) over 100 database queries to generate the forum's home page. Essentially they've done what they have been doing since version 3.8: piling way too much new code on top of old, inefficient legacy code. (I had to keep a heavily modified vB 3.7 for another "big board" just to make it play nice with the traffic we had.) IPB (Invision ProBoard) is about the only other choice that is similar. And the problem there is that when they do version upgrades, it breaks just about everything--themes, plugins, etc. Others have far fewer features (Burning Board, Vanilla Forums, etc.), and the freeware versions (having used most of them personally) are sorely lacking in many ways. I run four other forums that us XF and have no issues whatsoever--best forum software I've used. There are some minor features missing, but mostly on the admin side of things. Each situation is different though. And the alternatives are such that XF was a default choice for the forums I've worked with.