A good question indeed. I was toying with the idea of upgrading my 40EX720, maybe to a 46NX720, or even a 55HX820 (at a very good deal).
Spent some time again in stores, to observe if the extra price was worth it. Especially, check the colors, as I could read that the NX and HX were somehow better than the EX in this area.
Had the 3 sets aside, and honestly, I couldn't see a big difference. All sets were set in dynamic mode (sigh -as often in store), and I found that the 55EX720 (PVA) in demo was performing fine against the others.
Now, I also read that the NX720 had excellent blacks, and this could make a big difference if watch movies in a dark room. The EX720 is just fine in this area.
Personnally, I always keep an ambient light when watching TV in the evening, so I am not sure the improvement would worth twice the price.
Attached to my message, some exemples pictures:
1-55EX720 (left) against 55NX720 (right)
2-46HX820 (left) against 55EX720 (right)
3-my 40EX720(PVA) at home
Obviously, no conclusion can be made from theses pictures (especially with this low quality), it is just a very relative overview.