It's important to note that every Penance tick has travel time, this means that if target is close to you, ticks will hit the target quickly, while if the target is far away from you, it may take more time for the ticks to hit your target. This applies for both offensive and defensive usages of Penance.
Due to its mana efficiency and healing speed, it is generally the primary direct heal used by Discipline priests.
Even if a pushback prevents the final tick from landing, this is still a more mana efficient heal than [Greater Heal], and will still heal more than [Flash Heal]. Pushback can only prevent one tick, as it can only cancel 50% of a channeled spell.
To successfully fake cast a Penance, the best thing to do is to cancel Penance right after the first tick has been cast.
Patch 5.3.0 (21-May-2013): Penance now deals 10% less damage, but healing done is increased by 10%.
Patch 5.2.0 (5-Mar-2013): Evangelism's stacking effect now also increases the healing of Penance.; Penance damage and healing has been increased by 20%, now a cooldown of 9 seconds (was 10 seconds), and has a range of 40 yards for both friendly and hostile targets (was 40 yards for friendly and 30 yards for hostile targets).
Patch 4.0.6 (8-Feb-2011): Penance mana cost has been increased by 7%, but healing has been increased by 20%.
Patch 3.2.0 (04-Aug-2009): Cooldown increased to 12 seconds, up from 10 seconds.
Patch 3.1.0 (14-Apr-2009): Penance can now be cast on the priest himself and the damage has been increased.