You sould burn your drain lifes and S pains primarily, you really dont have better uses for them, not critiquing just asking you opinion on it.
Yeah, Soul Burn Drain Life is really bullshit. I always use it as aff. 15,000 damage in 1.5 seconds (if you don't crit any ticks) and 15% health is pretty amazing. I'd have Soul Burn SP'd during the mage duel but locking shadow school locks out Soul Burn. I really think Soul Burn should be off school lockouts. :( Although Soul Burn Searing Pain or SB Soul Fire is better for Mage duels because Mage Armor is BULLSHIT STILL.
I use SB Soulfire as Demonology mostly, it does a buttload of damage and is good to fire on the run, and I pretty much only use the other effects if my pet dies or if I'm in a PvP scenario with more than 1 opponent. I sprint a lot in world PvP though, its really helpful kiting if someone (a pesky rogue or warrior) is sticking to you...or to chase some pussy Alliance down and execute him.
If you play it right, its pretty much impossible to lose as Demonology. The amount of damage you can put out with just Metamorphosis is pretty extraordinary. Stacking Demon Soul+Meta and jumping onto someone, then Coilstorming them solo is almost a guaranteed kill within or shortly after the Death Coil if you have DoTs up. I'm absolutely IN LOVE with Demonology right now. Its fun and its more difficult than the other specs to play well and it feels just right in terms of "tools to use" in PvP. Felguard hits hard and feels like the pet it always shoulda been. For you vets its kinda like how when patch 2.0 came and FG was hyper OP. He hits haaaard as fuck with Mastery. I think a lot of you will really like playing it (if they don't nerf the balls off of it first).