I tinkered with TV audio to headphones before.
In my case, I used a variety of headphones (AKG, Bose QC, etc). Then I also put a receiver in the middle, too (because my TV is part of an HT setup). The receiver has a headphone jack and I can make some sound adjustments further. I'd done this when watching dialogue-heavy movies late at night.
Now, the answer to your question if the TV audio is good via headphones is a bit quirky to generalize, BUT one needs to consider at least that the source material is also good. Are you simply watching crappy TV broadcast from local channels? Satellite cable with HD channel? Or are you watching bluray movies?
The rest is subjective. Just like the other previously posted replies, the sound quality rests on your personal preference. What may sound crappy to X may sound golden to Y.