Well yes.. at least for the beginning and if the service really does a great and GENUINE job. It will make your forum look running, busy and active. YOu can continue the service until real users replace those paid users nicely. But if you have already many supporters and friends who can refer a quality userbase, it's equally good and longlasting.
These guys seem to do a job. ForumsFirst - The Premier Forum Paid Posting Service
Didn't try them myself but heard of them from a friend long back. I'm gonna use them for a niche health forum though. If you wanna wait, I'd love to give a review Else, try them with the smallest package and write a review here too.
Hope this helps.