I never cared for the sets in diablo 2. It was kind of neat that they were there, but I was far more interested in mish-mashing my own set of gear out of unique and rare items.
I especially hate sets in diablo 3, mostly because they are the most powerful option for every class. I really think they detract from the ability to customize a character or be creative with them. It's a very inlined and hand-holding way to say "this is the best gear, use it"
of course, sets don't have to be the best gear, in which case I wouldn't mind. I'd still rather more resources went into unique pieces of armor/weapons than making full sets, though.