We've done it twice, once rt in 05 and again in 06. $600 for two people? That's actually a better price than we paid in 05 for coach. We did a roomette in 06 and we prefer coach because the roomettes are small and cramped. We slept fine in coach. We're probably going again this fall for our cruise and WDW trip. The seats in coach are about the size of first class airline seats and there is plenty of leg room as well as overhead storage. They separate families with small children into cars with other families, and seniors (snowbirds) tend to be in their own cars. My DH and I, both in our 40's traveling w/o children were in the car with families with older children and singles/couples. It's much quieter than the family cars. There is a thread on here with tons of info on the Auto Train. It's under the sticky-"Click here for popular transportation links".
Also, if you don't qualify for one of the few discounts Amtrak offers, you won't find a cheaper fare. The fare is the fare whether you book yourself or go through a
travel agent. Fares can actually go up the closer you get to the trip.