I trust estibot more than any other tools in my toolbox. At least for the search numbers and other info. As far as appraisals, it is not bad for an automated service, but that is not what I use it for.
But I really prefer Wordtracker as it has stats for singular and plural. Makes difference, singular isn't always best. For example cell phones got more searches then cell phone.
Also Yahoo is more US based so you got differenet peoples mentality stats then average of whole world so it might be contraproductive for some keywords and countries.