Like so many other search engine optimization agents, we, too, have found all ranking checkers we tested, including the market leaders, to be highly inaccurate at times, though not consistently so, which of course only makes matters worse. And which is why we finally resorted to writing our own server based, customized solution.
However, when determining a web site's search engine ranking, there's another fairly important issue to consider as well: search engine index flux. Take Google as an example: depending on whether you check rankings on "google.com", "www.google.com", "www2.google.com" and "www3.google.com" *and* depending on which phase in their ongoing reindexing run you
happen to hit, results may differ dramatically. (This applies to linkage checks, too, btw.) There's actually only a fairly short phase in any given calendar month when results on both "google.com" and "www.google.com" will be consistent. The situation is quite similar with
AltaVista, Inktomi and other contenders.
Traffic load, load balancing measures, indexing runs spread across hundreds if not thousands of web servers (Google is famed to work with 10,000+ Linux boxes!), ongoing ranking jobs, time of day, etc. are all factors which may skew results or make them fairly unreliable, especially in a scenario where you may have to illustrate a given ranking when presenting your
services to potential clients.
This is something SEO clients should be educated about very early on in the process to avoid possible loss of credibility and confidence.