Natural SE ranks or PPC?

Are there certain sites that you would rather use a PPC campaign for over natural search engine optimization? I realize for arbitrage, a cheap PPC campaign is best, and affiliate marketing (though I have not tried that route yet), but how about for service sites, bidding directories, forums, and the like?

SEO traffic is the best. It is highly targetted and conversion rate is way better than any other source of traffic. I prefer it over any other method to generate traffic.

__________________

That makes sense. I guess it would be worth it also to put the hard work in to get the natural hits so it is consistent rather than only when you run a PPC campaign.