It's great for finding alternative keywords/phrases and it also displays aprox. daily searches for each of the variations.
But generally the Wordtracker results are more accurate than the Overture results since it's pretty easy to manipulate them and Overture doesn't distinguish between plural and singular searches.
Aside from overture, I also use Google's keyword tool, formerly available only to Adwords subscribers. I rely more on google than overture though because it's a lot of work optimizing content for both SEs.
Those mentioned are the common and better ones. Its sometimes best to go with 2-3 keyword tools. I can suggest another here - http://freekeywords.wordtracker.com/
You can try it out. Do let me know how you find it.