The basic idea for this build is to use talents for instant Corruption with the Felguard. It is advantageous to specify full improvement of Fel Intellect as well as Fel Stamina in combination with Demonic Knowledge.
As from patch version 2.0.10, Demonic Tactics will increase crit chance by 5% instead of spell damage with 5%. In effect, it will decrease the damage output for this build by 5% because DoTs don't benefit from critical hits, and shadow bolt is rarely used in this spec. The only things benefiting from Demonic Tactics are Felguard's melee crits and Seed of Corruption detonations. In both cases, crits only increase damage by 50%.
Use in PvP
A good PvP build based on the Felguard. Two instant DoTs are available (Curse of Agony and Corruption. The strategy is quite easy with this build: Let the Felguard attack and DoT anything visible. Instead of Shadow Bolt, use Drain Life. Crit chance will be low, but isn't needed.
Use in solo PvE
Felguard/affliction is probably the best build for soloing 5-man instances. Felguard is good at holding aggro and surviving while your DOTs do damage. Health Funnel allows you to heal your pet very efficiently, prolonging the fight. For optimal results, put 2 points in Improved Health Funnel and 2 points in Improved Curse of Weakness. Moderately geared level 70 felguard-spec warlock can solo a lot of surprising things -- from Felguard Elites in Tainted Scar to all of Scholomance to the 1st boss in Underbog.
Use in raid PvE
Felguard is highly resilient in Molten Core, BWL, ZG and AQ20, including the ability to DPS most bosses and nearly all trash. Somewhat fragile in AQ40/Naxx.
DoTs + Drain Life will do very high DPS so long as there are a reasonable number of other warlocks present to assist in obtaining a reasonable number of Affliction effects (10+).
With the same gear, overall DPS can match heavy Affliction and surpass level 60 Destruction based builds, while remaining entirely self-sufficient due to substitution of Drain Life for Shadow Bolt.