I absolutely LOVE getting my car detailed. But I haven't been able to justify the cost in a very long time, especially since having a larger car, which of course drives up the cost. They basically recondition every "detail" of your car, using toothbrushes and qtips to get into crevices, shampooing the carpets, hand waxing the outside, maybe even buffing out scratches, etc. It is great when you have kids. Oh, and pets usually add to the cost as well if there's odor or dog hair embedded in carpets, etc. Generally in the spring I'll detail my car myself, or I'll go to a car wash that does a la carte and do a little bit at a time (though frankly, I do a better job myself, lol).
I would clean, but never spend the money to detail a car before selling it, though (unless I was selling it privately). Dealerships employ people solely for this purpose. Besides, they're looking at the bones of your car, not it's condition, which is generally fixable, or at least improvable.