Hi qwestzander,
I've just recently flown from Rome to NYC and return. One way with LH and the return trip with UA.
All I can say is that I refuse to fly UA ever again, unless my life depended on it, in which case I probably would fly them :P
There was absolutely no comparison in the two airlines. I flew Economy with Lufthansa and it was still so much better than Economy Plus with United. Most flights via Munich with LH have the newer planes. Often, connections in Frankfurt use the older ones. So it's easy to check.
I give my LH flight 7/10 and my UA flight 1/10. I've never flown AC, CO or LX, sorry!
Daniel.