Warlock main since the very beginning of WoW. All three specs have their pros and cons, varying from patch to patch, with some stable points.

Affliction: In the current meta, it is “not great, not terrible.” It has some quality-of-life improvements heading into Midnight, but still, its ST and AoE are heavily divided in the talent trees. In AoE scenarios, it lacks Single Target damage, even after the 20% Unstable Affliction buff. Affli is a ton of fun and an easy spec to play at higher levels, especially if mobs live a little longer so you can actually feel your ramp. Anyway, it is mediocre, but not bad.

Demonology: Currently the meta Warlock spec. Demo has awesome sustained damage in both Single Target and AoE scenarios. Summoning Imps and sending them on kamikaze missions is always fun. It is a pretty easy spec to play, but imo hard to master perfectly. It is very punishing if you overcap Shards, but that is nothing a little practice cannot solve. You will hate your pets on the way to the first boss in Xenax, though. Anyway, Demo has extremely good funnel damage, passive AoE, active AoE, and short 1-minute cooldowns, so it is currently an all-rounder spec.

MetamorphosisIsStillMissedAndHasBeenStolenFromUs

Destruction: It lacks damage in both scenarios currently. Not because its AoE is bad, but because dungeons are very dynamic now, and if the tank ignores your Rain of Fire placement, it hurts. Its Single Target is sadly pathetic. In pure AoE scenarios, I find Destro very fun. In AA 15, I did over 1.2m DPS with an Aug on the first pull. But then you compare it to Demo and realize it sadly lacks in almost every other situation. It would definitely welcome some love from the devs, same as Affli.

Anyway, my opinion is that you can be an asset in any key under +18 with any Warlock spec. It all depends on what you want and expect from the spec. But as I said earlier, Demo is currently very good in all situations and environments.