The answer is easy... Everyone who is buying a house needs an appraisal! Make sure your agent/or lawyer puts into the contract to have the offer contingent upon appraisal. You don't want to be locked into a purchase price that is significantly higher than the house is worth. Having an appraisal done tells you how much your property is worth today in it's current condition. Whether you are getting a loan (where an appraisal will be required) or you are paying cash get an appraisal done!
Especially in this market where home sales may be not as plentiful in the neighborhood you're buying make a professional appraisal essential. Any buyers agent worth their salt can give you a rough estimate of value based on a CMA (Comparative Market Analysis) but this is not to take the place of an appraisal.
The appraiser inspects the property and surrounding property to see if it's on they're comparable and makes adjustments if they are not. They can also determine the value of the property based on replacement costs. Appraisers use a variety of techniques to determine value, but the bottom line is thatpaying several hundred dollars is pretty inexpensive in compared with buying thousands of dollars too much for your next property.