Just blogged about the benefits of text link development over buying banner ads: Marketing Your Online Presence: Banner Ads or Text Links The long and short of it? Go for organic search results by developing text links, not buying banner ads.
I will agree, banner ads are not that effective when it comes to link building. They can bring some traffic depending on where you are advertising.
I liked the blog post. It really seems like there are a lot of people who aren't considering what they're getting in return for their web marketing efforts, and an equal number of them willing to sell them junk because of it (inflated banner ads, expensive PR N/A directories, the list is long.)
instead of paying banner ads for link building. try social bookmarking,blog posting and forums.they can bring good traffic.
I hate banner ads, they always have payed out very poorly for me. Usually they are CPM though, so if I had a ton of traffic they could serve a purpose.