Other specs use Death Strike to heal when necessary and not as part of their usual damage rotations (e.g. if the healer is out of mana and cannot keep you up) or, if having chosen the blood specialization, to generate Death Runes when needed. Its main purpose is to heal, not to deal damage.
While tanking, it may be best to wait until you take damage before using Death Strike; this will make the heal worthwhile, and spare your healer(s) some time and mana.
Patch 6.0.2 (14-October-2014): Death Strike now causes healing that scales in effectiveness with attack power, instead of based on damage taken in the last 5 seconds. This healing is affected by Resolve.
Patch 5.1.0 (27-Nov-2012): Damage received from players (or player-controlled sources) no longer counts towards increasing the healing done by Death Strike.
Patch 4.3.0 (29-Nov-2011): Death Strike now heals the death knight whether or not the attack misses, or is dodged/parried. As a result of this change, Death Strike no longer refunds its rune cost if it fails to hit the target, as the death knight will still receive the healing effect.
Patch 4.1.0 (26-Apr-2011): Death Strike self-healing no longer generates threat.
Hotfix (2009-04-21): "Death Strike will now heal the Death Knight for 5% of his or her maximum health per disease on the target, up to a maximum of 15%. "
Hotfix (2009-04-17): "The healing a Death Knight receives from damage done by Death Strike has been reduced. " Healing changed to 5% of maximum health + 5% per disease, up to a maximum of 15% with 2 or more diseases.
Patch 3.1.0 (14-Apr-2009): Increased damage to 75% of weapon damage (up from 60%), reduced healing by 20% to 80% of damage dealt + 40% per disease (down from 100% of damage dealt + 50% per disease).