A few years ago I branched out to UA (back in the day when CO was in SkyTeam...<shudder>) and like you I was at first disappointed by the hard product offered.

While I'm not 6'4" (~6'1"), I too had an issue with that #@$ headrest, especially on takeoff as it would always jab me in an awkward way. The trick I found was to leave it down but angle it forward so it would provide support for my upper back/lower neck for takeoff and landing, and then as soon as the wheels lifted off the runway I put it up.

The tradeoff I also found with the headrest is that when it's up, I've found it easier to sleep with - especially if I've got a pillow that's escaped Jeff's cost-custting-clever-o-doom.

Anyways next time you're on a UA flight, try fiddling with the tilt (not the height adjustment) and you might find a better position for the headrest.

Beyond headrest issues, the best seats IMHO are the old CO FC ones -- man, those are/were great! Liked how the seat fit my back (which granted is a personal issue), liked the little spot to store stuff in the center armrest (UA sorely lacks a similar space), and I like the fact the drink tray extension in the center armrest goes forward vis-a-vis to the side on UA, so you can use it and your tray table at the same time.

Shame they dropped that seat as the new one they're putting on the 739ERs just does not do it for me.