You must have come across ads that promise you to give you visitors for cash. I have never tried that because i believe its not worth buying. These ads are the other ends of the ppc or gptr sites. This means the visitors are members of these sites who are payed to click on links. You just buy the packages of how many visitors you need and asap you get those. But such visitors are of no use in an interactive site, or even a normal site/blog. The reason im saying this is that the visitors they send you are money oriented, who have joined those ppc or gptr sites to earn money for themselves and have no interest in your site. They will just come visit the home page, the next moment surfing another homepage ;). If you are buying visitors for a forum or a social networking site, its of no use !

If you are concerned about your rank only than it may benifit you in terms of pagerank,alexa rank or so on. But if your site is running on ad revenue, such visitors wont even bother to look at you ads ! So if you are interested in just visitors i would recommend entrecard is the best way for driving traffic (that too for free!) instead of wasting money on buying visitors. Genuine traffic (coming from sources like search engines,blog communities, etc) are better, as they will be one who will bother to read your post and comment on your blog (also to click on ads :P).