I really like it like emergency AOE. When you feel overwhelmed by numbers just place it and activate Also blocking doors is funny. Or using it as a kind of decoy: especially one time I did it great when I didn't have enough AP do destroy goliath a placed it in front of him and next round killed him. Or as a barrier in front of turrents in few places where static defence was an option. Otherwise, if no good use is available just put it behind you and kill some of your pursuers.
Definitely great as a tactic to slow down and kill pursuers havnor
Slowing down Goliaths is also a great move -- he can waste his attacks on the mine, and if you get lucky the mine will keep him busy an extra turn before blowing up. Help the mine out a bit by shooting him lol.
I find them most useful when defending a tact point. I'm limited in the number of fixed turrets I can have, so I drop a land mine next to any known spawn points . The spawned xeno will attack the mine first, and give me time to rush a templar over to solve the problem.
I use both turrets on overwatch and land mine as "speed bumps" to buy my templars enough time either to rush over and deal with the problem or to escape. While I haven't used it this way yet, I might also consider using the talent when having to traverse a large map with an engineer in tow. Leave a present behind me so that pursuing xenos will stop to address the mine rather than simply charging after us.
For both of these occurrences the actual destructive power of the mine is unimportant. What is important is their ability to force the xeno to slow down and deal with them, potentially buying me a turn or two.
Landmines allow an Engineer to fill in for a Overwatch Soldier/Neptune to a tolerable degree. Together with Sentry Turrets, of course. To summarize and expand a bit on what was said so far: - Their primary use is as a decoy to protect the flank, often in an ad hoc manner, when there is not enough overwatch or when the enemy is too near (e.g. a heavy hitter closer than 3 tiles, or a ranged unit that will be able to shoot at you, or a stratos turret). Obviously, you don't want to detonate it manually in this case. - The napalm they leave is only 5 squares, but it hurts a bit. Not bad in a choke point if you don't have a Hydra on the field. - They can be manually delivered grenades too, especially with weak enemy units (cultists). In this case, drop it in a middle of group and detonate it. - Or they can be a safety net in front of a defensive line. Just make sure that when they detonate you don't have a turret in their napalm.
Landmines are useful at low talent levels (3x for L1, 4x for L2). Considering how many great talents Engineers have, that's pretty much the highest I go.