I have now ridden the train in the US three times: this past Saturday, this past Monday, and this past Tuesday, all portions of the distance between Washington D.C. and Durham, NC.
This gets a five because the seats are great. They're huge, with plenty of legroom, and the train itself is amazingly smooth except for when you're decelerating just coming into a station. I'm someone who can normally never read in cars or (often) in airplanes, but I had no problem reading here.
Not so hot. The train is awfully slow (5 hours for a trip that should have taken 4 in a car), and was 30+ minutes late arriving two of the three times I rode. Also, train stations are pretty scarce the farther west you go - the East Coast is the one part of the country where it makes sense to have Amtrak run. (You can take the train to Duluth, MN, for example, but not Houston.) On the East Coast, though, it makes a lot of sense, especially in areas where driving would be a headache with lots of traffic and rush hour. Also, you don't have to wait around for two hours before leaving like you would at the airport. (Extra bonus: CD players and laptops aren't forbidden.)
$50 round trip to Washington D.C. is a great price, and not all that much more than you'd spend on gas in a standard SUV non-fuel-efficient car. Plus you can read or play games on your laptop instead of having to drive. It gets a four because if you're carpooling for shortish distances you would certainly be farther ahead to drive, assuming one of you has a car.
Better than airline food, but not by a lot. It's nice to be able to get a snack or lunch whenever you want, though. The pizza was scary (think microwaved pizza-in-a-bag), but the sandwiches were pretty good, and the prices weren't any higher than you'd find anywhere else with a captive audience (movie theater, amusement park, etc.)
Clean, and tastefully decorated, and well-lit even at night. The stations varied, but were clean (at least the ones I saw) and I didn't feel unsafe standing around at night. You get a good variety of people taking the train, but there were a good sprinkling of college students and people in suits, and nobody smelled like they hadn't had a bath in a long time. A few little kids, but they were more cute than annoying (at least the ones that I saw). I certainly didn't feel like someone thought I was a potential criminal (e.g. when you have to get your shoes wanded separately at the airport), and the conductors were all polite and friendly and helpful, even to the annoying guy who hadn't bought a ticket beforehand.