Nice thread TAY, and you hammered home a key point. Unpredictability and the element of surprise are usually the defining features that win matches between two evenly matched players. While these unconventional sets tend to lose their effectiveness over a period of time, they can be very, very effective if you don't flaunt them unnecessarily. Sometimes, getting your opponent to think something can be vital to a set's success, even if it is something as small as running Shuca Berry over Leftovers on a Body Slam / Iron Head Jirachi.
Anyway, here is a 'gimmick' set that Imran and I have been using lately in UU, 'BaitLix':
Steelix @ Passho Berry (water resist)
252 Atk / 252 SpD / 4 Spe
- Iron Head
- Stealth Rock
The team we ran it on had problems with Water types, in particular things like Slowbro, Milotic, and Azumarill. Steelix, by virtue of its typing and defenses, lures these pokemon in, as they are some of the few that can actually hurt it. The Passho Berry allows it to survive a Surf from Milotic or Slowbro, or a CB Waterfall from Azumarill, even if it is at 50% health. This means that it can reliably take these pokemon out with an Atk invested Explosion.
One more thing - the term 'gimmick' is often thought of as being a derogatory term for a set, but really, all it can mean is that it is a set that is very team specific, and so does not deserve to be in an analysis, etc. Like the BaitLix set - it is far less effective than a normal defensive Steelix at taking physical hits, being 2HKOed by things like a Hitmontop Close Combat, which Steelix would normally take with ease. It is less effective at the job it is traditionally seen as doing, and instead, it has been tailored for a team's specific needs.
I think this is the exact same way that the 'bulky CBers' work. No one expects Donphan or Swampert to be doing any real damage as they are usually seen as bulky tanks - because that is the job they do best. You can really catch your opponent off guard and get some surprise KOs if you slap a CB on them and really abuse those 100+ attacking stats. But of course, this would mean that they won't work as well defensively as they normally would, and so you have to balance it out and see if the surprise value and extra damage is worth the loss in defenses.