I definitely don't think Shadow Priests are op. In fact, I think they are in dire need of retuning.
1) we have no burst CD (aka personal CD unless you spec Power Infusion). Virtually all other classes have at least 2.
2) We are a DoT class, and our DoT DPS is a joke. Even at lvl 90 we are mathematically lower than an affliction lock.
(good post on the issues with this: http://us.battle.net/wow/en/forum/topic/6308612711
3) Orb generation takes at min, 30 seconds to fully charge (due to the CD on MB). In a pvp battle, 30 seconds is a long time, to build up then hold your DPS burst. This means we cannot use DP or Psychic Scream lest we lose our orb build up for burst damage. I'm not even going to talk about SW:D in terms of Orb Generation because at 20% hp (when SW:D is usable, you should be bursting, not generating orbs, so that mechanic is totally useless on SW:D, plus it has a 6 second CD...
4) Overall DPS is very low.
5) Our 2 non damage breaking CC has been reduced to 1.5 CCs - Silence being the best choice but only affecting casters. Psychic Scream is shadow orb based (on duration) and forces orb consumption. Other classes have at least 2 forms of CC where the only restriction is the CD. We have a CD AND require orbs to use this. Sure we have fear, but now other classes have more fear mitigating abilities. In order for fear to be beneficial you have to chain it in the beginning to force trinket pops, spell use (to put them on CD) then recycle them again after your CD are up. That makes the first 30 seconds of the fight for you pretty much only doing DoT dps (which stated previously is low).
6) SW:D...complete joke in terms of DPS and kill mechanics. 6-8 second cooldown on an ability that you can only use when your opponent is at 20% health. It's a finisher that doesn't have the power to kill unless it uber crits or your opponent is at 10% and frankly by then, dot's and mindflay or an instacast mindblast would finish them off.
Yes we have fear chains, but when your DoT damage is low, and you have no effective burst damage to finish the job. You have to rely on that gimmick to happen repeatedly. LOL chain fear lock anyone? That's already been proven to be a novice strategy and easily prevented with a well timed trinket and silence/fear. There are just way too many fear mitigating abilities across all classes to make this a solid pvp strategy. Basically the only people you'll be killing with this strat are inexperienced pvp players or just plain getting lucky because your opponent didn't have his CDs.
Lastly, I think this class in general has lost its feel. It doesn't feel like playing a shadow priest anymore. It feels more like playing a discipline priest back in Vanilla when the disc tree was lol. I suspect many of us old timer SPriests are going to reroll affliction locks. I sincerely hope blizz comes to their senses and fixes this class. It's the first class I ever played and I still play it (despite the fact that I think it's broken - in a bad way.)