I personally cannot find anything to recommend it, and I was posting the exact same thing in Hubpages earlier...if you don't mind me simply linking to it?
hubpages.com/_39d9hjsh86gzq/forum/topic/50803
The main problem I have with it is that once your money dries up then so do their posts, and if you are not large enough to survive on your own without them with revenue generated then buying traffic will not sustain you.
In order for this to work you need a category domain preferably or at least a strong one, like bodybuilding.com where a seed 1000 people can work wonders.
If you are making affiliate sales then buying a good few months of posts from people can work until you start to get natural traffic flow, but most people cannot afford this. It can get very expensive very quickly. For instance, in 2007 you would be looking at between $0.10 and $0.20 and if you need 1000 posters to do 100 posts each it is going to cost up to $2k each month, any less and your forum will not achieve critical mass.
Critical Mass being the phenomenon where more people are joining every day just because others have joined, like Facebook. Once that got to 1,000,000 users and everyone heard that on the news the population exploded.
Many is the tumbleweed that blows through forums where someone bought traffic but could not sustain anything, because as soon as his paid posters disappeared there were only a few people wandering around like that guy who always gets left at the restaurant to pay the bill with a dumb look on their face. (That actually does happen...)
If you notice, any niche has either one or two leviathans dominating such as Name Pros for domain names, Warrior Forum for internet marketing, The Admin Zone for webmasters...if you are trying to break into one of these markets then you are going to have to make sure you can offer something that no one else has.