Lots of good information so far.
I've taken Amtrak many, many hundreds of times, like 2X or more per week for ten years.
For day travel, trains on the NEC (North East Corridor) are fine. The seats are wider than coach airplane and with more depth between them, more like airline business class or first class seating. Since you are only short time, the cafe car is adequate. Acela trains are nicer, but not worth the big bump up in price. (While my employer would have paid for it, I typically didn't use Acela because of the price difference.) Even the longer 'day' trip trains (see below e.g. Maple Leaf, Adirondack, Pennsylvanian) are fine in coach.
In term of long distance travel, I've taken the Silver Star (NY<->FL), Silver Meteor (NY<->FL), Lake Shore Limited (NY<->Chicago), SW Chief (Chi<->LAX), Coastal Star lite (LAX<->Vancouver), Pennsylvanian (NY <-> Pittsburgh), Capital Limited (Chi<->Washington), Empire Builder (SEA/POR <-> Chi), California Zephr (Chi<->San Fran), Maple Leaf (NY<->Toronto), Adirondack (NY<->Montreal). My advice for OVERNIGHT trains is that unless you are a teenager or in your twenties, you do not want to spend overnight in coach. Many times the trains are cold, people talk, you share bathrooms with the masses, and so on. My rule for overnight (after doing ONE overnight in coach) is never again (as long as I can help it).
Roomettes are tight, but I have done many trips with DS without issue. Eating in the dining car (as compared to the cafe car) is a nice experience. You might have to share a table with someone, but that in itself adds to the trip experience. The bedrooms are a little bigger, but also typically require more $.
One of the downsides (as already mentioned) is that some of the Amtrak employee's seem to forget they are in a customer service business. Also, if you take a train (especially a long distance train), you need to make sure that you allow for the train to be late, sometimes a lot late. When I plan a trip, I never ever ever have another connection on the same day I am arriving. (For example, if one were to take the train from NY to FL to take a cruise.)
Enjoying the train requires one to think about the trip in a different way. If you are going on vacation and you do air travel, then the airplane ride is a necessary evil to get to where you want to go, and you want to minimize the time there. If you have that attitude about the train, it will likely be an unpleasant experience. If, on the other hand, you see the train ride as a part of the adventure, where you can spend hours looking out the windows at little towns and while going through the country side, it becomes an enjoyable experience.