I generally like to watch TV with the lights off since it feels more like a movie theater, and it focuses all attention on the program and cuts out other distractions in the room, as others have said.
...unless I'm eating at the same time and need to see my food so I don't end up spilling it everywhere, or working on paperwork or other chores with the TV on in the background. Then I will turn on a lamp or two, depending on how much light I need.
Having the TV being the only light source in the room doesn't bother my eyes, since I have the TV set to "cinema" mode with lower brightness levels. At the store, an HD TV is typically set to "dynamic," but I've heard that it wears out the electronics to leave it that way all the time (or something like that). And I like the warm, "cinematic" look of "cinema" mode.
Was there any merit to the old story about watching TV in the dark damaging your eyesight?
Like Kirk, I have a fondness for antiques, and I often come across "TV lamps" from the 1950s that are meant to sit on or near the TV so there is still some light while the TV is on.
Kor