The way I look at it is that each greatest hits album is gonna have at least two or three of the band/artist's good tracks that will give a good reflection (assuming of course the band/artist is any good in the first place). Therefore as Shane said they are a good way of sampling a band to see if you want to here more. I personally have loads of greatest hits albums for bands that I hadn't heard much of beforehand.
Another thing is that it is most likely that if you buy a greatest hits album then you will no at least one song, if not its probably been a recommendation, in which case it must be a reasonable reflection otherwise you would have been recommended an ordinary album.
Anyways thats my opinion for what its worth.
P.S. I am not after a valuation on my opinion
