The question of this thread is "argon worth it?" That question goes beyond whether or not argon does a better job than air at keeping you warm. The question is whether that extra warmth, assuming it exists, is worth the extra cost and effort required to use it. I generally dive with a drysuit in three situations--open freshwater sites with temperatures in the high 50s, ocean sites with temperatures in the low 70s, and caves in the low 70s. I have only done occasional dives in the 40s, but they are actually the most instructive for this discussion.
I used to use argon with some frequency at the freshwater sites, but since we did not have a good way to bring it on extended trips, I used to use a big bottle and top it off with air as the weekend went along, so I was really using "airgon." In addition, without a booster, as the supply bottle pressure dropped, so did out argon fill ability, so we were once again forced to top off with air and use "airgon." Frankly, I never noticed a difference on those dives, and I eventually just went to straight air. I assume that the reason I didn't notice a difference is that I did not flush the suit as described in post #8.
What I have learned on those dives is that if I use my medium weight underwear, I am usually just fine using air for dives in the 70-80 minute range. On longer dives, I get cold, so on those dives, I put on my heavier weight underwear and stay toasty warm. On the dives in the low 70s, both ocean and cave, I am usually perfectly comfortable with my medium weight underwear on dives in the 70-80 minute range. The exceptions have been when I have had to work harder than normal in a cave, in which case I tend to get uncomfortably warm, especially if I wear a hood.
My few cases of colder water diving are most instructive. They were not really cold--about 46° F, but I made the mistake of wearing my warmest underwear and a hood while inflating with air. By the end of dives lasting for about an hour, I was sweating. I certainly did not need to be any warmer.
So, the moral of my stories is that if you have the right undergarment for a dive, there is no need to use argon, so that extra cost and effort is not worth it. I have never dived in truly cold water, and I have never done extended time dives. In those cases, I might very well become interested in using argon, but nothing like that is on my schedule these days.