I was a subscriber to Estibot for around 18 months.
Compared to other automated appraisal tools, Estibot is by far the most complete and comprehensive software that exists right now. As a developer with speciality in AI, I believe it would be very difficult to build a tool to outperform Estibot.
That said, it is by no means a good appraiser, if you compare it with a veteran domainer. A computer software is by its very nature quite limited. Its valuations are based on past sales, search volumes, CPC and other metrics, and *anyone* based solely on those factors is unlikely to make accurate appraisals. Whereas an experienced domain investor will take into account a hundred more factors, and his life experience - which can not be quantified and used by a software.
Estibot fails miserably on most brandable domains (since the typical metrics like search voulme are irrelevant for brandables) and it used to mis-evaluate .ORG's (not sure what happens now, the last year .ORG and .NET took a hit). It has other shortcomings too, but I won't go too deep.
The appraised drop list is as good as the appraisals

. Since it is not practical for a domainer to do a one-by-one appraisal on thousands of dropping domains, Estibot offers a shortcut. Then it's up to you to decide which domains you're going after.
Now about the end-user feature, I have used it a few times, sadly it wasn't of too much help. Several advertisers were out of business (so I guess the data must have been old) and I believe a simple Google search is better (1-2 first pages).
If you are just starting out, Estibot will likely help you. When you feel more confident, you can ditch it :D