On various admin communities, a whole lot has been said about the usefulness (or non usefulness) of promotion sites like Forum Promotion). Some people seem them as rather useful for the extra services and links, some see them as a waste of time because of the audience they draw in. But honestly, here's what I think about them in general.
They're really better for a very limited subset of the forum market. As in, for certain topics they work, and for a lot of others they just don't.
Basically, if your site is about webmaster stuff (like coding/managing websites), general chat or perhaps technology on a very general levels, promotions forums are pretty much right up your alley. They have the audience you want to reach, you'll get plenty of good offers for affiliate swaps from the people posting there and all in all, you'll probably get the most out of them as a result.
But if your site is in a very different market on the other hand... you're basically screwed. Running a site about a certain game series that has a slightly different audience to that of Forum Promotion or The Admin Zone or what not? Like my own 3DS forum, or a Mario forum, or a Pokemon forum? Good luck getting much out of it in terms of affiliates or active posters, because this audience tends not to use said forums like the webmaster community does.
The same goes for various other fields. Sports? Better hope you're running a general chat or football (either type) forum, because the audience just won't be there on a promotion or admin forum otherwise. A forum for pets? Good luck promoting it, the overlap between pet forum goers and tech geeks isn't a massive one. Same with other topics like a mums/dads forum, a storm chasing form (sorry Shawn) or an outdoor pursuits one. In fact, ask yourself this one question before you get your expectations up too high on one of these sites:
Is my forum about the kind of subject that would mostly be of interest to 16-27 year old computer geeks with a side interest in running websites?
Because if it isn't, promotion forums (and maybe even admin forums in general) won't really help a whole lot when it comes to marketing the site. It's true, I've seen it happen myself.
So that's my view on promotion forums. They have their uses if your forum is either extremely general or most 'tech' based in the webmaster sense, but they're most certainly not that great if you need to promote a forum with a specific audience that isn't really represented among their userbase.