Amtrak in the NEC is fine and PHL to NYC is great, WAY nicer than the fucking Greyhound albeit not cheap especially if you don't book in advance. I second anon, it was comfy when I used to take the route a few times a month. Not even first class required, the regular Keystone was fine, the seats are massive and you have so much leg room.
I took 1st class on an Acela once (I had a free upgrade from their loyalty program) and it was neat (nothing crazy though). Free snacks, hot towel, lunch service (if you wanted), and a more modern carriage.