Hey MCAD64, I too had the 7.1 bug. So that's what I have now. VP150 (center), M60s (fronts), QS8s (side surrounds) and M2s (rear surrounds). Coming in as the 20th or so person to put a post on here a lot has been said already. But I'll post from my experience.
It's true, there is no 7.1 "discrete" signals (though DD-EX could be argued as 7.1 discrete)...but all movies can be made 7.1 movies. Anthing can be upconverted from a 5.1 or 6.1 to 7.1. 5.1 to 7.1 spreads the sounds of the left surround between the left side surround and left rear surrounds, and visa versa for the right side. In 6.1 the rear surrounds play in mono when in a 7.1 setup.
I could go on about what I think about 5.1 vs. 7.1. But I'll sum it up pretty simply here. If you have a large room....go with 7.1. It will really help fill in the sound from the rear. I've done tests between 5.1 and 7.1 and I can't help but like the 7.1 better because the sound surrounds me more than ever. Like a movie theater you have the speakers on the sides of the seating and behind. But if you have a relatively small to medium sized room then 7.1 is overkill because you don't need that much sound being spread all over the room. Again. 7.1 has the side and rear surrounds share information. So unless you can really spread you speakers a good distance apart, it's a waste of money really.
Though Dolby Digital EX does (in some senses) use 7.1 in a disrete fashion. Movies such as Finding Nemo and Star Wars Episodes I and II all use DD-EX, as well as the Lord of the Rings movies. But, if you're going to watch Lord of the Rings I'd recommend the DTS 6.1 Discrete channels. So...there are a few examples of movies where having 6.1 or 7.1 would be nice over 5.1. But again, it really comes down to room size.
Small to Medium = 5.1 or 6.1
Medium to Large = 6.1 or 7.1
Larger than Large = how ever many amps and speakers you can afford
