No automated service will ever be dead on. It's just no way to account for so many little things that only a human can account for such as the typo domains, quality of traffic and things like that.
However as far as those automated services go, it is pretty decent. However keep in mind that Estibot was created for professional domainers and uses wholesale criteria to formulate its values and that may or may not matter to your specific situation.
What Estibot is good for as someone else already point out is to give you a solid starting point, but know that never in a million years are you likely to actually get what it says your domain may be worth.
For example it gives TaraPatrick.com (the typo for TERA) a value of $130,000 and on these GFY boards we know the guy is still trying to get around $2,000.
http://estibot.com/results.php?domain=TaraPatrick.com
But in the end I woudl say it's still a great free service that gives you a quickie snapshot of the domain.