I have a rather large room, but I have to agree with Geoffcin on this one...I was very skeptical at first, but an home demo helped me make up my mind. I use the processing most of the time for TV or movies to take advantage of 7 speakers. There are som odd ball exceptions where I can't hear a difference, or don't like the difference I hear.
I think Sir Terrence summed it up though, the benefits decreas in smaller rooms.

I'd recommend getting a handle on 5.1 before adding more speakers, though.