They can be highly inaccurate when compared to the actual number of searches performed for particular keywords. The tools will help you to select which terms to target, but they won't help you accurately predict the amount of traffic you can achieve from using those keywords. For that information, you can use programs from Google's AdWords and Yahoo! Search Marketing.
WordTracker and Overture also have other limitations of which you need to be aware. WordTracker separates terms and phrases by capitalization, plurality and word ordering. Overture does not.
Plurals do sometimes deliver different results in the search engines.
Changes in word ordering also deliver different results. :!:
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