Depends on where you are going - for my family vacation a sleeper for 4 is cheaper than flying by a lot (i live in a high priced air travel city, I would money driving to chicago despite the higher parking costs). However I have 5 people going and so it does't work out. (It doesn't help that amtrak dosen't suggest options like 2 rooms)

We went coach amtrak which was cheaper and more comfortable than flying. I'd do that again.