I think the fares are reasonable. I regularly travel amtrak between NYC and DC/PHL.
Living in Detroit, I can fly to NYC, take a meeting, hop on the train to DC, take another meeting, then fly home from DC. I certainly wouldn't drive. Also, if I have several days of meetings in NYC, but have one in PHL mixed in, I can go to the meeting and come back to NYC the same day.
And in terms of the cost, it costs me nearly $25 to drive 100 miles in my car, and considering I can't work while driving, it certainly is much more cost effective to take the train (and then you don't have airport hassles).