I've had my EMS kit inst. on my truck for about 20K miles, and I think they are great.
I'm sure I gained about 1 MPG, but my major reason for the EMS was the better bearing set-up, and not having to spin all drive parts in frnt. of transfer case. IMO that is just wasted wear and tear for nothing.
There is plenty of info. on this site about EMS and Dyna-Track, but I'll have to say EMS was great to work with when I installed mine.
Ray