As long as you understand that there is no real First in domestic US travel, except the magical LAX-JFK routings, DL does a decent job. Most of my domestic US travel is ~2-3 hr flights and I'm not willing to pay a substantial amount more for DL's domestic F, but occasionally it's made sense on the longer routes. I'm sure DL has some FAs who are worthless, but I've always had pleasant experiences with them, no complaints.

I actually don't mind the Mad Dogs (MD88/90) aircraft as long as I'm closer to the pointy end of the plane. In the back between the engines is miserable, but up front you'll only hear the air moving across the skin of the plane rather than engine noise. Sure, no IFE, and WiFi on them is rubbish, but it's a solid aircraft. Even Economy on one isn't bad as long as you're closer to the front and in one of the "2" seats of the 3-2 configuration in Y.

Also, if you're booking F, I'd say avoid the domestic-configuration 767-300 (non-ER). I've had better C+ seats on a Mad Dog than the F on that particular 767-300.