Since i was lv 60, i tried Demo, Affliction and SL/SL.
In general, i found that Affliction is the quickest on killing multiple targets, Demo (with FelGuard) kill single target quickest and SL/SL has the least down time.
In BG, Demo is very good against spell casters when getting use of Intercept, Fear and Death Coil. SL/SL has a good survival ability and slightly better dmg (just DoTx4 and move on another target). Affliction got DoTx5, but i found that when fighting in many to many battles, when i finish landing the 4th DoT, the enemy is dead. (or i m dead =_=; ). May be i m stuck on my play style, i found SL/SL is sightly better in balancing.
Most of my friends in Guild are DemoLock . Only 1 is DestroLock (we call him King of OT) and some are AffLock. SL/SL build is quite rare too.
In Raid, we found out that our pets die easily (except unSL phase-shift Imp). Both VW and FelGuard need to pay care on it's HP.
For both SL/SL build and DemoLock, i hit an impact, which is when my pet die, my DPS drop a lot. Anyone has any suggestion?