we have used SMF on the manchester regiment forum for the last two years or so, we have nearly 1200 members, 1.8 people joining and 1200 views per day, it's a very busy forum
we haven't had any problems (yet!) no spammers to speak of, no spambots at all. but more importantly, it's free! you have to pay for hosting your site, which iirc is about £20 a year and your domain registration, probably a fiver
it's a damn sight cheaper and just as good with better security (IMHO)
chris
edit, anyone interested can have a look at the manchesters forum, it's open to guests. i won't expect you to understand whats going on in the military and family history but you can get an idea of how it compares to this one