There's considerable debate over whether second and third tier search engines are worth you advertising dollars. Some point to user experience and quality assurance concerns. However, there is also the argument that most of the traffic being taken by these search engines is referred through affiliate sites, so if the second-tier search engine has a high quality affiliate network (i.e. no spammers and polices click fraud), then it might be a good place to invest your PPC advertising dollars.

After all, affiliate marketing portals subsist off of targeted traffic. If the affiliate is a stand-up businessman and not just a doorway spammer, then what they're pretty much doing is pre-screening the traffic for you. In other words, the clicks you'd be getting from advertising on the second-tier search engine that's powering these affiliate sites are coming from users that have already typed in a couple different keywords combos into Google, were subsequently directed to the affiliate site, and are conducting what is, for all intents and purposes, a vertical search. That is, they already know what they're looking for. It follows, then, that second-tier search engines that have high quality affiliate network (and not all do), conversion rates may very well be better.

You see, Google's main strength can also be one of its weaknesses. Essentially, it's so large that search results are often overwhelming. For example, most users will start off their searches on a top-tier like Google. Depending on what they're interested in (free info or spending money), from the SERPs, they'll likely explore a number of both organic and sponsored results. Then they might try several variations of that first keyword.

If the users goes on, then, to conduct further searches on a site that was, itself, in the SERPS of Google (or one of the other top-tier engines), it is likely because they feel that this site is a specialized vertical. In other words, their search on Google leads them to a niche site that, for the purposes of the vertical of that niche, they perceive to offer more targeted and specialized information.

If you share this line of reasoning, of course, the question becomes: how do I select second tier search engines worth advertising on? Well, fortunately, Gordon Choi is offering 6 tips on testing PPC search engines. Here they are summarized, with a bit of my own two cents following in the brackets:

Set Expectations
Never expect the lower-tier PPC search engines to perform better or even similar to the top search engines. [That doesn't, of course, mean that they can't...]

Set Priorities
If your products (or services) are in a specific industry (or niche), you may want to first consider spending on PPC search engines that are in your industry. [After all, the strength of second-tier search is targeted (or pre-screened) traffic...]

Get the Data
If the PPC search engine ranks your ads by using a metric such as Clickthrough rate (CTR = Click / Impression), then it makes sense for the search engine to provide the data so that you can analyze the performance on all levels (campaign, adgroup, keyword and even ad). [...]

Test the Best Keywords and Ads (Only and No More)
You may be wasting your valuable time by dumping your entire Adwords keyword list into a smaller search engine, [If second-tier search engines are good for certain traffic verticals, then obviously your entire keyword list may not convert well on any given one of them...]

Detect Click Fraud
Use a click fraud detection system if possible. [Many second-tier search engines claim to anti-click-fraud measures as their strength, and while it is very true for some, it's complete bulls**t for others...]

Make Tough Decisions
You can only test before you find out. [Part and parcel of the process is preparing yourself for the reality that experimenting with some second-tier engines are going to be a waste of money, but you just have to make a choice if you're going to be sure that you're not missing out on a valuable online marketing opportunity...]

Gordon, of course, goes into more detail than I do in the original post, so if you have the disposable PPC advertising budget to test out some second-tier search engines with, I'd suggest checking out his post. Remember, however, that depending on their affiliate network and partners, some engine are going to be a gold-mine of conversions, and others are going to be disastrous. So before dropping some cash on one of them, see what you can find out about the quality assurance of their affiliate network.