Having worked in-house for several large online retailers as well as a handful of independent clients on the side, the comparison shopping engine industry was my obsession for several years. CSEs can make you money, but it doesn't take long to figure out all CSEs do is play a game of "sell traffic to retailers for cheaper than they can buy it themselves." After 12 months of CSEs, it was obvious that being a "CSE expert" wasn't a good career path for me.
Arguably, Nextag's four competitors are Google Shopping, Pricegrabber, Shopzilla, and Shopping.com. Google Shopping was free to list your products for the longest time (until earlier this year), which provided for a much better user experience. Between 2008 and 2010, Google Shopping's marketshare kept eating away at these other CSEs. It was clear their product was better on all fronts.
I think it should also be noted that in 2005, Shopzilla was acquired by Scripps for $525M. Scripps sold Shopzilla to a private equity company for $165M last year. Pricegrabber was bought in 2005 as well - $485M price tag. They just got dumped for ~$80M.
Most CSEs were founded in the 90s; you could argue that their models haven't changed since. It's not working anymore. Take it with a grain of salt, but peek at Google trends and Nextag's Alexa site info:
Nextag's services are becoming obsolete. I find it concerning that their response is to "double its spending on Google paid search advertising in the last five months". I guarantee you they're seeing the same kind of traffic patterns in Bing as well.
Nextag is just a whole bunch of click arbitrage silliness. Don't drink the kool-aid.