I two boxed a shaman/mage, but changed to lock/mage when the mage hit 80. My guildies (my kids, haha) are farming heroics and raids with this combo. Nice DPS with a mage/lock combo.

I've got a small team of 3 locks sitting at 29 waiting for their rested experience to cap. They deal out some damage. I easily do quests 4 levels above me. 3 voidies tanking mobs while you dot, tap, drain life... repeat. No real need to buy water, but you can't quite effectively drain tank at this level. I can't wait for siphon life next level. Drain tanking should become a lot easier then. When my main (a lock) was 70, I could just about drain tank the entire Charred Rise. I made a ton of gold that way. So locks would be a good choice for fast leveling, especially in heirlooms.

I'm also 3 boxing a Boom/ele sham/ele sham squad. They are at 61. NPC's and Players alike die fairly quick. I've got all 3 knock backs on round robin. I drive with the boomer and use standard click macros for each. The shammies go OOM before the druid does.