Either because of a bug, or because of a flying dead corpse making it's way, but an enemy just killed by Mindfray relatively next to a one tile wide normal terrestrial door opens said door. You need to factor this into your plans because it opens new areas for your vision thus may activate potential new hostiles, especially dangerous in Iron Man mode.
If a target is wounded, there is a greater chance to succeed due the Will penalty inflicted on the unit.
As Mindfray doesn't factor in cover or distance, use Mindfray against targets that are in cover when your weapons have a lower chance of hitting them.
This is especially useful with Assault class soldiers who are unable to sufficiently close the distance to a target and have to wait for a turn in half cover or take inaccurate potshots with pistols, especially if lacking Rapid Fire.
Can be used effectively to instantly kill Sectoids, Thin Men, and Floaters without expending ammunition or risking a miss.
Mindfray will be your bread-and-butter ability for Psionic units, as a Mindfray and Mind Control combo can easily lower an enemy's Will and then quickly take control of them.
To do this, Mindfray an enemy, wait for the next turn, and then use Mind Control on it. A soldier cannot Mindfray and Mind Control during the same turn, since it takes two action points and requires one action point to take affect. There is also a one turn delay before taking control of the Mind Controlled unit; by then the enemy will be back to normal and lose the Mindfray debuff.