Receptional_Andy - 7:32 pm on Aug 11, 2008 (gmt 0)

I would second the opinion that if you're going to use one tool, then now that Google supplies (approximate) numbers then that's probably the way to go. But I'd also second the opinion that combining data is the most effective approach. I would put Microsoft's Live Search's keyword services platform as one of the important sources to use if you have access to it.

Note that the Adwords tool seems to include data from the content network - so it's skewed in favour of large content sites that drive adsense impressions, and this means that the numbers you see may not equate to human keyword searches at all. But, they have by far the biggest sample, which is something that's hard to ignore. Once you combine a few sources you dramatically reduce the potential margin of error.