They're useless when it comes to appraising domain value. I do like some appraisal tools like checkpagerank or smartpagerank because they give you a lot of other info. like reg. dates, archive history, backlinks, etc. I've found that the actual appraisal values are way off. I've sold domains for $200-$300 that appraised for $0 (because they were newly registered or had no backlinks) and I have domains that are appraised at $5,000 that I would have a hard time getting reg. fees from. You should read as much information as possible about domains, then use your own good judgement.