If you're looking for a good free PHP based forum script I highly recommend MyBB. I used to use SMF but once I switched over to MyBB I haven't looked back. The only cons of MyBB are that there aren't as many themes or plugins but the ones that do exist are pretty good. In addition, the support forum is VERY useful and a lot of the members on there will really go out of there way to help you which is great.

Any other MyBB fans or even haters?

I've never used (or, to be honest, heard of) MyBB; I've always used PhpBB or SMF.

My biggest question is this: how secure is it? Do its users ever have major problems with bots/spammers?

I'm concerned about security, too. Sure open-source is great, but without a community like that behind Firefox, I don't know how secure these smaller forums actually are. All open-source forums should merge into one to get a bigger developer community.

Until then, splurge for vBulletin if you can. It's much easier to admin from the CP.

More info on MyBB can be found here.

Haven't tried using myBB though. Will try to check it out!