I have worked on several systems now with the additional height channels available with Dolby PLIIz and Audyssey DSX. Based on listening to several setups these channels can significantly improve the sound field for movies. I found this most successful where the front main speakers must be placed closer together than 40 degrees with the height channels mounted above the screen and about 60 degrees apart. The additional height channels widened the apparent sound field to more accurately reproduce a movie sound experience with the left and right main channels anchored near the display.
This type of configuration may be very useful in a typical living room where the main channels are restricted to being next to the sides of the display while ceiling speakers or smaller speakers on shelves can be spaced much further apart.
I also found the Audyssey DSX implementation to be more pleasing to listen to than Dolby PLIIz. However, the Audyssey automatic setup was not very successful compared to a proper manual setup.