They are very gear dependent. Once you can get your stats bloated out they do fine to great, as previously mentioned, against certain classes. I played feral a ton in pvp in retail in vanilla. It isn't until BC where they become extremely powerful but in some respects they are more balanced here.
Indeed they have a serious weakness against well played shadow types and healers. Paladins are actually doable in long battles where you lay your bleeds out over and over but could simply stalemate. Battlegrounds or open world pvp is where they are meant to be (no arena here anyway). I usually would hang with a rogue or other classes that made up for feral's weaknesses against some classes. Always found it fun to play along side a good hunter and laugh at how many fools ignored me thinking I was their pet.
It is all about power shifting and being extremely mobile. Very fun if willing to accept their limitations in vanilla.