I'm gaming with an Epson 8100 on a 92" screen from 15ish feet away. Playing MW2 like everyone else. Given my age, I suppose you can consider me an adult
It's a great experience, a lot like watching a movie, the bigger screen really sucks you into the story. The scenes when you're running through a burning DC were amazing. I'm looking forward to Mass Effect 2 on the big screen.
Playing online is very good also, I don't know if it's the most competitive way to play however. Other players are more visible (larger) but you have to move your eyes or head farther to see all possible hiding spots. With a smaller screen you might be able to take a visual 'snap shot' and cover all the hiding places/alleys at once.
I'd like to go with a larger screen in the next few months, going to move my couch closer in the mean time to test.