The only problem with the 400 mm lenses is just the cost. I know for most of the people I talk to, dropping $1,300 to $5,000 on a lens is a bit much unless you're either very serious about photography, money is not an object, or both. You're right with the "buy cheap, buy twice" thought process but if you can't drop that much coin on a lens I think a 200 mm or a 300 mm lens is sufficient for most people.