I've been using Deo-S, and it's pretty great. I'm not entirely sure if it's broken exactly, but it's still very potent. For me, it mainly acts as an emergency escape button if something powerful is about to stomp your team, but can also wallbreak pretty well. He's one of the few Pokemon who can break through both Blissey and Gliscor with the same set.
Personally, I run him with 0 speed EVs. He has problems with scarfers, but he's faster than the entire unboosted metagame besides Mega-Alakazam. In return, both his attack and special attack reach 400 with Life Orb taken into account. He's basically a pokemon with 140/140/150 base offensive stats, but without an item and taking damage every turn.
Of course, that's probably not the best spread. But that's the great thing about Deo-S : you can tailor him to outspeed anything up to, and including, Scarf Garchomp, and create the balance you want between power and speed. You should probably pack Scarf Latios or Terrakion to deal with faster Scarfers. But Deo-S can hit harder than any Choice Scarf user, and can also switch moves. Not many Pokemon common in OU can take both a Psycho Boost and a Superpower from it too well, especially given the lack of viable Psychic-types. Steel-types fare decently against it, (it can't 2HKO Skarmory, Bisharp defeats it if it switches into anything other than Superpower and Aegislash is a great counter), but it really doesn't seem to mind getting checked too much. It's very much a hit-and-run Pokemon. It behaves much like a scarfer, except it isn't necessarily easy to check.
The only problem with it is that, for some reason I don't understand, it doesn't get Fire Blast. If it did, then I think it might be broken, but without, it can't deal with many Steel-Types and has to settle for the measly Fire Punch which is never useful except if you're facing a weakened Ferrothorn and don't want to drop your Attack and Defense stats.
So, overall, it's not broken, it's just a great Pokemon with a very solid niche. It's a bit of a jack-of-all-trades, which can both work as a revenge killer, end game cleaner, and wallbreaker while not being all that good at any. It definitely should get more usage than it does.