I’ve been using CommunityServer to power the forums at CSFBL for a few years now, with mixed results. The version I am running (2.1) is a bit buggy and a poor performer in some respects, but it is serviceable. Still, with hundreds of thousands of posts, I really need something more reliable.
The obvious choice was to upgrade to CommunityServer 2008, but problems with the upgrade script and stickershock at the new price tag pretty much put this right out of contention. I went out and bought a license for vBulletin — one of the most widely-used bulletin board products available — for $160, about 1/50th the price I’d pay to upgrade CommunityServer.
vBulletin offers some support for converting from CommuntyServer via their Impex utility. Unfortunately, the current implementation they had was quirky and unreliable… but vBulletin gives you the source code (everything, including the import/export scripts for dozens of other forums), so I began to tinker.