The guy behind NameWorth is a smart and very experienced domainer himself, Garreth.
The idea that he has is actually a good one - see how much a reseller will pay, an impulse buyer, an end user etc, e.g. not one price but the whole set of it, depending on who is buying and when. The problem is that it's just a very few basic parameters that he is considering at NameWorth (I am actually not sure you can set up more of them in one prog, and if some people have it - I see no point in going public with it for a membership fee), - and as a result the digits are very far from reality.
It's good to use it in your first few months or for a year, as exactly as mentioned - it boosts your ego and you understand how people come up with basic calculations for good names, because again - the core idea is valid.
But later you start using much more parameters, you understand brandables, niches, hyphens, you get other tools, you learn sales history, similar words, etc, - and then (and pretty fast!) you get to the point when to spot a good name you need no estimation service (none of them).
So, bottom line - if you are a beginner then nothing wrong with using NameWorth, but I hardly believe any experienced domainer is using it (or any estimation service at this point).
Good luck!