Do pay per click campaigns work for you? Unfortunately, PPC is very hard to see a definite benefit from when you have a content site that you sell advertising on. On the other hand, if you have an ecommerce site that is very goal oriented, you can very easily track PPC campaign results.
For this reason, I have always shyed away from ppc campaigns. Yahoo! just has never worked for me because I find their interface clunky and their minimum bid way too high. But with AdWords I average a conversion rate of a few % (with a conversion being a new registration). However, I find that visitors coming to my site from AdWords typically stick around longer and are more likely to post after registration, so, over time, I can guestimate that it's worth keeping it going.