Mines are (pun not intended) a blast. There is one new map, that has a large patch of cover outside of your base (it's a winter map...the center of the map is a lake or something)...I covered it with mines, and placed turrets outside of the cover. Orks charge in, thinking they're taking cover from turret fire, and get blown sky high and shot to pieces. I'll find the replay and post it...
There you go, mining begins at about 11 minutes in.