Do In-ground pools add or detract to a residential property's appraisal or market value?

Working in the ad valorem property tax m appraisal field, I have heard various comments from home owners that in-ground pools take away from the actual value of a home or property due to the known dangers of pools and the potential increase in home-owners insurance premiums. It's been my experience that this is not the case as most houses of similar size and in a similar areas or markets with a pool will sell for more then houses that are comperable with out a pool. This theory also doesn't seem to deter home owners from putting pools in anyhow. I'd like to hear from Real Estate Agents and any experienced Appraisers with their comments on this. Is there truth behind this theory or is someone giving some bad advise to home owners.