On rental cars ...
I generally use Enterprise. They are fine. This time Delta had "deal" with Payless. OK, saved $180. Got a Nissan Sentra with 74K miles and that stupid rubber band transmission. Not a fan. The car was "OK" overall, I guess, I never trusted it to stop when needed much. Usually with Enterprise I get a modest upgrade or three.
On First Class ...
Not worth it obviously. We were upgraded going out, which is "nice", breakfast was decent. I looked at paying for the upgrade, it was $329 EACH. Not a chance. Coming back we were in "Comfort Plus", I dunno about the plus. It's fine, a bit more leg room and recline. I'm willing to pay some more for that.
On "Premiium" ... found generally on overseas flights. It's nicer of course, but also pricey. I upgraded us coming back from Seoul to DFW and I guess that was worth it, my wife liked it.
On Delta One ... We had this once because the flight from here to Seattle was that sort of aircraft, just came in from Rio. I spent a lot of time playing with the gadgets. That was our only experience with it for obvious reasons. If a Comfort seat to say Paris is $1600, Delta One might be $7500. Each.