I bought one banner Ad in pr6 site,but that is nofollow link.so i wanto know nofollow is useful for my site pagerank? thanks.
It is not very usefull, if it was dofollow, as google can find out that it is a banner - you paid for link, thus your link wont count when passing pagerank.
It won't count for page rank but it's still a link worth having, if all your links are "follow" then it would look unnatural and Google could penalize you. best to have a decent spread of both do and don't follows.
With a PR6 - the site must have a ton of traffic. You should be getting plenty of hits from the ad regardless of the lack of SEO benefits.
Its useful in a way that your telling google your not doing blackhat but it will not help you rank the page/site.
Nofollow is not useful for increasing Page Rank as nofollow links do not pass on any page rank to your page. But the link will hopefully send some good targeted traffic to your page so the link will still of benefit to you.
Like others have said, probably not worth it for link juice purposes but if it's generating enough traffic to your website and it's cheap for that traffic then keep it going and people will link naturally to you anyway. Natural links are always the best option. Totally depends on your website though.
NoFollow stops everything. However, Google may still use the metric for link spread analysis, they have not confirmed either way. Trust is indeed non existant, it is a term covering a wide range of trust factors, but Google has confirmed that different values other than page rank are passed via links. Whether they do this for individual factors or for a calculated trust factor based on individual factors is unknown. But everything to duplicate content to the position of the link on a page can affect the value of a link.
It seems like a strange question to ask after having bought it. You should be asking these questions before you decide to give the site your money in exchange for a service. You get what you pay for. In this case, you paid for traffic passed to your site from his, only.
Page rank palys the vital role to drive traffic. Only "do follow" link can increase the page rank. So "no follow" link is good for nothing.
I wonder how many of the people here saying nofollow does not pass pagerank have actually run an experiment on that. I have one site that I only every linked to with nofollow links and it is a PR2 and on the first page for it's keyword. So yes, your PR6 link is good, but it won't pass the same juice as a dofollow IMO.