I look at the cost-per-conversion and compare that to the average cost of my product. Ultimately, I want the campaign to be profitable but I also recognize the inherent value you get from branding... so even if there are campaigns or keyword groups that are less than profitable (in terms of cost-per-conversion), you can still justify the spend.

More than the conversion, you get the branding for your products, ad and also the new customers into your fold (if they buy it from you). Once they click through your website for the keyword, you get better ranking in SEO also. While converting the cost, you should consider these factors also.

That is a good point. I was not aware that increased traffic helped SERPs as well. I knew it did pre-Google search engine days, like when I used Copernic and other small search engines. Thanks

It doesnt depend on your ctr or anything. It depends on the number of users your campaign generates and the number of pageviews you get and the amount of repeat visitors and the number visitors who subscribe or make a purchase. this is hoe you determine on the success of your campaign

Apart from the CTR, I personally believe hearing from the client that his business is doing well since his PPC campaign was put up is a gauge of the success of your work.