For warlocks from 8-62, this spell should always be maintained. Its long duration means you don't need to recast it very often, and it's usefulness is not to be underestimated.
After gaining [Fel Armor] at level 62, Demon Armor is oft forgotten. However it still has its uses:
The Healing bonus applies to bandages and Healthstones as well, so turning it on after a resurrection of any variety is both useful & efficient.
The added armor and health regen is useful in Multi-mob drain tanking of higher levels, a PvE grinding technique for Affliction Warlocks.
It is useful in PvP, especially for affliction locks due to its quality of enhancing both [Haunt] and [Drain Life], in addition to the glorious armor bonus.
It can also be used for lock tanking, which is useful e.g. against the Blood Prince Council in Icecrown Citadel as it increases healing received by 20% and more if it is specced with [Demonic Aegis].
Patch changes[]
Patch 5.0.4 (2012-08-28): Removed. Armor and self-healing bonuses merged into [Fel Armor].
Patch 4.0.6 (2011-02-08): No longer costs mana. Duration now lasts until cancelled, up from 30 minutes.
Patch 4.0.1 (2010-10-12): Now learned at level 8, down from 20.
Patch 3.0.8 (2009-01-20): Armor granted increased by approximately 120%.
Patch 3.0.2 (2008-10-14): No longer grants health regeneration or shadow resistance (HP5 merged into Fel Armor). Now increases all self-healing received by 20%. Tooltips updated.
Patch 2.4.3 (2008-07-15): No longer considered a magic effect and cannot be dispelled.
Patch 1.7.0 (2005-09-13): The armor increase will now stack together with Scrolls of Protection.
Patch 1.6.0 (2005-07-12): Now increases health regeneration at all times, as was always intended. Previously, the health regeneration only occurred out of combat. Tooltip updated.
[Demon Armor].
[Fel Armor] at level 62, Demon Armor is oft forgotten. However it still has its uses:
[Haunt] and 
