As I was leveling up my warlock, I had imagined myself raiding as affliction. I have always enjoyed the play style of affliction and don't mind making things a little more complicated for myself if it means I can dps at a higher level. Once I got to 80 and tried it, however, I realized just how complicated afflictions rotation had gotten and started to rethink my plans. It was very difficult as I was getting back into running 5-mans as ranged dps, to concentrate on where I needed to stand and what I needed to target all while trying to keep 5-6 DoTs on 2-3 mobs and wondering what mischief my felhunter has been up to since I hadn't been paying him any attention (sometimes, if I ignore him for too long he leaves a little surprise in my PvP boots). This prompted me to check out what specs the other raiding warlocks in my guild were using. To my surprise, they were all using the Fel Guard/Emberstorm build. All of them. This got me to wondering if I should give it a shot. As much as I love affliction in theory, I love having the Fel Guard out in practice. Ever since I saw his cowling face back in the first 2.0 patch, I've had a special place in my heart for the big lug. He's been through all my leveling from 60 to 80 so I think he deserves to see some real action, other than just being around for a bit when I was asked to lock tank Leothoras the Blind in SSC (the only time I ever spec'd Demo in BC raiding). I have to say, after a few weeks and 2 wings of Naxx cleared (Arachnid on regular, Plague on Heroic) I am pleasantly surprised at how well this spec currently works.
The basic idea is that you have the felguard and most of the spell damage boost of the Demo tree and then add in the fire buffing of the Destro tree all the way up to emberstorm, which is a fire damage boost that procs off of shadow spells. You cast your two insta-shadow dots (CoA and Corruption) to proc emberstorm, then cast Immolate which will boost your direct fire damage spell, Incinerate. After that, it's just a matter of refreshing dots, spamming incinerate and making sure you click your demon's extra crit ability (I forget what it's called) when the CD is off (once per minute) and you're good to go.
I think the thing I like about this spec is it gives you a good taste of just about everything in the tree. You get the best demon who is out and doing damage instead of sac'd, you are refreshing 3 DoTs (unless you're assigned Curse of Elements which only needs to be refreshed every 5 minutes) and you are throwing regular direct damage spells. It's everything that a lock is rolled into one spec.
Now, with Patch 3.1, they are throwing in the Soul Fire talent that, when the target is below 35% health, you'll get a proc that reduces the casting time more than in half and removes the soul shard cost, meaning that we'll have another button to push between incinerates right when we need to push our dps the most. Plus, they are giving our demons more survivability though a 2 point talent in the easily accessible top of the demo tree that will heal our demons by 30% of the damage we do ( ! ). For those of you paying attention, that is similar to the t5 set bonus and will keep those pets up through just about anything. Also they have thankfully reduced the cost of a talent that allows your demon to gain mp from life tap from 3 points down to 1 making it very attractive and will help keep your pet's dps up where it needs to be.
I cannot emphasize enough how much I am looking forward to the new patch as they take my current favorite build and make it that much better.