as a druid main over several expansions (since tbc) and a pure pvp player with a very high experience and ratings in 2s and 3s i will tell you that ferals never felt weaker on any expansion so far than they are on mop's 5.4. why? mangle bleed buff exists no more (30% increased bleed damage), no more instant cyclone anyhow (the most important thing in the current meta is the ability to instantly cc someone, since here, every single class and spec has some kind of an interrupt and you can't afford to fake cast or recast cyclones as you're losing a lot of uptime) - both nature's swiftness and predatory got removed from feral. damage got nerfed from 5.3. as well, the only difference being is rip being buffed by 20%, which is still way less than with mangle. overall, the spec is fine for dueling and world pvp. however, someone WILL mention this, so i'll just say it anyway, sodapoppin managed to get a gladiator title in 3v3 as fmp (iirc), but then again, people aren't really playing 3s and some other comps are just way higher in tiers anyway.
overall, if you care about ganking, 1v1s, world pvp and stuff like that, then go for a feral. otherwise, go for the monk, since they're way higher in arena tiers than feral druids and were even played at blizzcon, what can give you an indication on how good the class is.