Minimal Value...
I am Diamond and will be next year. But fly almost exclusively US and Canada and find that since I don't like the uncertainty of not getting an upgrade I always purchase 1st class tickets, being diamond has very little value except:
1. Faster accumulation of miles which I am pretty good about using
2. While an unofficial perk, I have not been charged a change fee this year... maybe 3 times I needed to make a change.
3. The coolness of it.
I flew a couple of United flights because the price/schedule was way better but the program itself is not particularly endearing. Hate American so would not go there, and I hear about as many people hate United so not thrilled about going that direction. If Southwest added first class I would be all over that.