In terms of impressing me personally, the Meta / Ruin build I went with has a lot going for it. The playstyle is all about shadow-based nukes, and the role-player inside of me will never be truly happy with any fire based build until they turn the fire green or something. Normal, Mage-colored fire just isn't infernal enough for me. Unfortunately for this spec, it hit me during a bad week for my group. So the only testing I was able to do with it was during 10-man Sarth with one drake, Arachnid Wing in 10-man Naxx, and a speed-run of Karazhan that we did for the lulz.
I think the most important thing there is to say about this spec is that it's fun. A lot of fun. I've always been a strong advocate of complexity and depth in a rotation, but the simplicity of this spec really charmed me. My crits were frequent, and absolutely huge. Fully half of my crits were 10k or above, and many of them broke 12k--though of course the largest ones were when I was in demon form. And speaking of demon form, what could be more fun than turning into a giant black-and-purple badass every 3 minutes?
It felt really good to be in a Shadow Bolt heavy spec again. That's the kind of spec I was raised on. My first character was my Warlock, and he was destruction until his mid 50s. As much as I love affliction, shadow bolt spam always feels a bit like coming home. And I was even happier to see that Meta / Ruin is nowhere near as mind-numbingly boring as the old 0 / 21 / 40 spec was. But enough reminiscence! Lets look at some hard numbers.
With this spec, I had:
Health: 16,700 (Improved by stat food)
Spell Power: 2243
Spell Haste: 411 (12.53%) (Improved by both Spellstone and Stat food)
Spell Crit: 27.46%
Hit: Not capped.
Silly me, I recently upgraded two of my pieces. Ones which had a great deal of hit rating on them. It isn't such a huge deal for me normally. I just switched some talent points into Suppression and Cataclysm and I was good to go. It didn't even occur to me while I disenchanted my older pieces that I wouldn't be able to get those talents in some of the specs I test out. Oops. Fortunately, these days I run with both a Boomkin and a Draenei. The combined 4% hit bonus the two of them were able to provide for me kept me capped. But it is worthy of note that if you're going to spec M / R, you're going to need the full 446 hit rating. (Of course, you could just rely on certain buffs being available to you in raids, but I personally prefer to be completely self sufficient.)
Tactically this spec is very simple. The felguard does his melee thing, while you pound the target with shadow bolts. For added DPS, keep Corruption and Curse of Agony active on the target. Those two dots counted for roughly 25-30% of my overall damage. Demonic Empowerment should be popped almost any time it's available, for the added DPS it gives your felguard--who accounts for a significant portion of your DPS.
Metamorphosis on the other hand, has a pretty rough cooldown on it. Most boss fights are so short that they're likely to be over before you'll be able to use it a second time. So in most fights, I'd use this ability as an anchor. Pop it when the group's DPS needs to be at its absolute strongest. On fights where the boss has a soft enrage at a certain percentage, that's about when I would pop Meta. Or, on the Sarth plus 1 drake fight, I used it as soon as Tenebron showed up to help the group get him down as fast as possible.If there's a Shaman in the group using Heroism or Bloodlust, then you should probably pop Meta at the same time, so you can get as much damage done while transformed as possible.
In regards to the felguard, I decided to take this as an opportunity to learn from the commenters on my Fel / Emberstorm post. In particular, it was mentioned that using a Scroll of Strength VIII and Kibler's Bits on the felguard significantly increased the pets DPS. And, after buying some Scrolls and stealing some Kibler's Bits out of my girlfriend's bank (she's dedicated to the cause of Warlock science) I settled in to do some DPS testing.
I performed my tests against the normal level 80 test dummies, since I was unable to find out whether my hit rating affected my pet's ability to hit. (I assume it does, but was unable to find confirmation) I started with a control group, to gauge my felguard's unbuffed DPS. Then I worked with just the Scroll, and then with both tthe scroll and the pet food.
Unbuffed: ~520 DPS
Scroll of Strength VIII: ~555 DPS
Scroll of Strength VIII & Kibler's Bits: ~580 DPS
I'll admit, this isn't the leap in damage that I was expecting. But it's important to note that the DPS of this pet is rather variable. Each test was about 60 seconds long, and I had one that showed the felguard at over 800 DPS. I wasn't able to replicate that test, though, so I think it must have been a lucky string of crits. None the less, if you're serious about playing this spec (or any felguard spec) at endgame, then keeping yourself stocked with scrolls and pet food is definitely going to give your damage a little boost. Personally, I think I'd stick to only using scrolls myself, unless I had the recipie for Kilbler's Bits or Spiced Mammoth Treats, with ready access to the mats. Otherwise they just seem too difficult to come by. I spent quite a bit of time searching both the auction house and trade chat, with no luck.
The felguard is only a fraction of your overall DPS though. What's really important is how this spec fares on its own. And to be honest, I have mixed feelings. The initial few rounds I spent against a test dummy were certainly promising; my DPS was higher than it was either of the other 2 specs I tried before this one. I also managed to rock the damage meters when a few friends and I decided to go back and run Kara. But training dummies and outdated raids aren't the measure of a good spec.
Sarth plus one drake didn't go quite so well for Meta / Ruin as I thought it would. As fantastic as the burst damage is, it requires the ability to stand still for a little while. And needing to dodge waves of lava, then switch to killing adds, then switch back to the drake, then switch back to the adds again while dodging lava makes for very little in the way of casting time. Obviously you've got to expect a drop in DPS in a fight like that, but dropping below 2k DPS isn't something I'm comfortable with.
Still, as fun as the spec is to play and as much promise as I had seen in it, I really wanted to prove it could be competitive. So I broke my rule and I took my demon form to Naxx. We didn't have our usual offtank that day anyway, so I figured achievements were out of the question. Less running around definitely seemed to boost my damage, and I averaged out to about 2.2k in the first wing. Not bad, but not great.
The Warlocks in the top-progression guild on your server are probably never going to be specced Meta / Ruin. It's not that kind of build. Meta / Ruin isn't a perfectly optimized spec which squezes every single possible point of damage per second out of the class. Anybody who would criticize this spec because of that is missing the point. This spec is for people who want to be good enough to raid in less prestigious raiding groups, or even people who just want to PuG a heroic now and again. People who really enjoy throwing shadow nukes, or who just think demon form is really cool.
Bottom line is, if you spec Meta / Ruin, you've gotta be perpared for a lot of other Warlocks smuggly waving their DPS in your face. But all you have to do is pop into demon form and smugly remind them that you look way cooler than they do.