[Zyonin]

Speaking from experience from Battlegrounds, Arenas and World PvP plus Raiding. I have fought many warlocks (and lost) and also have a Warlock alt myself that sees PvP action.

Demonology is a good spec for PvP as that Felguard can make life hell for squishy dps types. I have lost track of the number times I have been charged by a Fel Guard (charge stun) then got beat on while the Warlock is DoTing me up. Good for World PvP or Arenas where the encounters are swift and groups are small or not existent. Demonology is also a good build for Leveling from 55-70 as you have an excellent tank and damage dealer in the form of your Felguard (who at this point has a few Grimores upgrades) This spec is also good for Heroics in that your group has an additional tank and DPS. However this not so good for Karazhan or 25 man raid as most tanks want the Blood Pact buff. However if your guild already has an Affliction Warlock you may be able to stay Demonology and bring your Fel with you.

Affliction can also be a good PvP spec as your DoTs get buffed and Corruption (with talent) is an instant cast. DoT, DoT, Fear, DoT, Death Coil, Fear, Dead oppenent. Affliction is a good build for Battlegrounds as often you will be with a group of people plus fights can last a lot longer allowing your DoTs do their full damage. However with an Affliction build you will want to team with other players (Feral Druids, Rogues, Priests are good to pair with). Affliction is an excellent leveling build for the 1-60 grind and is also an Excellent Raid spec. Most Warlocks that I run with (Level 70 Druid speaking here) in Karazhan and Gruul's Lair are Affliction.