In recent years I have purchased the higher end Yamaha receivers and/or integrated amps that used the presence feature first with the Pro-Logic soundtracks and later for DD and DTS. As far as I could see they were really the only ones that have continually had this feature. It is basically a sound overlay on the front L/R, surround and rear surround speakers to provide more depth and expand the soundstage and also gives you the capability of manipulating the effect to taste(delay, room size etc.). As far as the front L/R, you have the option of integrating the presence sound into these speakers without even having the extra set for the front presence effect. Yamaha's latest flagship receiver(Z11 $5000) has the 11.2 function which provides separate amps, in varying power levels for 11 channels. That is if you want to max out the capabilities, have the space and no WAF to worry about. A number of companies have had similar presence features, but you could only use them on two-channel stuff. Yamaha gives you the capability of overlaying the presence feature over "any" surround system.
I have found it to be a very worthwhile feature in acoustically dead rooms, especially for movies.
