G's keyword tool is probably accurate, but what many people don't consider is the fact that not all of the searches are done by consumers. Tons of searches on competitive keywords are done by marketers who are doing niche research and by those who are just checking their rankings. If everybody used the AdW0rds Preview T00l (which doesn't count any of the searches toward the monthly search volume) to check rankings instead of G itself, then the G keyword tool would more accurately reflect the numbers of "legitimate" searches.
As for WT, sometimes the results closely match the actual number of visitors that you get for a keyword (assuming that you rank highly for it), while other times it's way off. What I sometimes do is take my list of keywords from WT and plug it into G's Traffic Estimat0r. Yes, this tool is meant for estimating AdW0rds traffic. However, it can be used to gauge how much traffic you might get if you rank in the top 10 for the keywords in question.
For example, you might find a keyword in WT that shows 100 daily searches, but the TE might show that you'd get only 2 clicks per day in ad positions 1-3. Since position 1 in particular tends to get the most clicks, this result implies that there just isn't that much traffic available in G for that keyword.
If, on the other hand, you find a keyword that gets a lot of clicks per day, then you're likely to get even more traffic if ranked #1 for that keyword. Even if you're not using AdW0rds, the AdW0rds tools can be helpful for guiding your SEO projects.
Tools mentioned:
Code:
AdWords Keyword Tool
https://adwords.google.com/select/KeywordToolExternal
AdWords Traffic Estimator
https://adwords.google.com/select/TrafficEstimatorSandbox
AdWords Preview
https://adwords.google.com/select/AdTargetingPreviewTool
Wordtracker
http://freekeywords.wordtracker.com/