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.
Ive found that terms with around 2000, country specific, and exact match switched ON (most important) that its quite normal for me to get about 75% to 85% of that in clicks when at position 1 or 2 in the serp.
Exact match is the key to getting it so its not overinflated (in my experience).