I have been reading alot on this issue and from those that have tested these height channels, the majority of reviewers were quite surprised at how "large" it made the sound from the front appear. But as mentioned this depends on the source material. It should also be noted that most who did review this, immediately wanted to add this nifty feature to their home systems but also noted that although it gave a much larger sound effect (during rain scenes for example) that it was not needed in a properly set up theater, but was just a "cool effect". As mentioned this does require two more speakers.
About eight years ago I read an article written by Tomlinson Holman (of THX fame) who was at that time trying to sell the concept of 11.2 surround sound (four height channels - two in front and two in back added) but all the places he gave a demonstration to where quite impressed but not interested because they felt at the time the general public had a hard time going from stereo to 5.1 sound and they surely wouldn't be ready for 11.2 sound.
3db, yes Yamaha has had this feature for years, but Dolby managed to "perfect" the sound from the height channels better than Yamaha could, which yamaha has admitted is more of a "simulated" height channel whereas the PLIIz does a much better job of retreiving the sound and outputting it out those extra cchannels.