FranticFish - 8:50 am on Jun 30, 2011 (gmt 0)
They wrong at both ends of the scale.
For more popular terms they can be exaggerated by a factor of 10 sometimes.
Similarly for long tail terms if you see -1 or 0 interest, doesn't necessarily mean that either.
Use it as a guide, but the most accurate keyword data you can get by running an AdWords campaign. As long as you can afford to be on page one for a month (and your spend doesn't always have to be massive to do this) you'll get far more accurate numbers.