For me, I've always used 7.1 setups. But I've done so due to the fact that my HT's have always been so spacious. The bigger the area around your seating area, the more speakers you'd need (I would think at least) to help fill all that space with sound. If you're in a tiny 12x12 room. 5.1 should be plenty. But in my HT it's around 30x30 with open areas to other parts of my home. I've tried 5.1 and 7.1 in this room, and I'll stick with 7.1 all the way.
Also you need to keep in mind not only space, but placement limitations as well. You don't want to just have a speaker randomly thrown into a certain area, if it's not going to be within reasonable distance of where it should be properly placed.
For me, I plan on upgrading my AVR this year once I see what the new 2012 models have to offer. I'm planning on making the move to a 9 channel set up to try out what having heights can add to the equation. I have 3 spare channels on my external amps for this. Also in my ceiling I am able to get access up above for the wiring and I can ideally place the heights where they need to be, so that I can get the most optimal effect out of them.
I've wondered about how advantageous having wides are. I mean after all, if you're watching a movie, it's common to hear sounds to the rear, to the sides, and up above to an extent, but how often would you hear sounds from way over to your right side, or waaaaaaaay over to your left?
But if I had that option, I could definitely see myself trying out wides just for fun.
It's just that my right wall is not far enough away for this to be an option.