Finicky to say the least... I've been playing with displacement maps on various types of images to see the results... it seems to boil down to how much detail the image has and how the map wraps around the image... and what the image is..ie-- a persons face , an object , a land scape etc.. One thing I do is to de saturate the image then use levels to brighten and darken it without loosing to much detail. Once i have what i am looking for then i use gausen blur so that the map dos not create very sharp edges when applying it and gives the subject a better coverage, even after using a map i would in some cases use the warp tool after it to bend around a nose of person or rounded shape object to get the final result.
I'm getting more and more use to Photoshop's built in tools and how to use them, and practising helps to keep down looking for a quick fix by using resources I no longer need as I get more use to the tools and how they work.
I mention in another thread how powerful the brushes built in to Photoshop really are, i do not need to down load them as much as i used to, People should take the time if they have it to get to know the power of the built in tools and how you can use them.
Just the other day i wanted to warp part of an image where the shadows did not fit just at the right angle on the floor. so made a selection of the floor and half the image and found out i could actually warp this area once it was selected. Thats probably old news to some but for me it was like finding gold for what i wanted to do.
Any way just keep shopping and practising.