Diane, while I love and use the boil down technique, I certainly wouldn't call it a substitute for crystal. The results are quite different. IMO, each has it's place and they are not interchangeable.
Yeah, you'll get different results. Think about the difference between a malty, but relatively dry North English Brown (Like Sammy Smith's) vs a typical sweet American brown ale.
Reduced wort will provide depth-of-flavor and color, and will provide more malty/caramel flavors the darker you cook it, but you'll never get the sweetness you'll get from crystal malt. I agree with Drew. It's appropriate whenever you want a more robust malt flavor, or flavors in the same "class" as crystal malt, but don't want the residual sweetness.
I don't want to get into a semantic argument, but the difference between a caramel reaction and a Maillard reaction are often confused in food industry literature because of their similar chemical/sensory characteristics. There is a ton of flavor overlap between the two.
Chemically speaking, caramelization: sugar + heat = sugar loses water, undergoes further dehydration. Maillard reactions: sugar + heat + amino acids = sugar loses water, undergoes further dehydration, plus some other stuff happens.
I make a lot of caramelized sugar syrups for my Belgian beers, and I've gotten lots of crazy flavors out of them, including crackers, toast, roasted almonds, and other things that don't taste like what we typically think of as "caramel" flavors.