I have run Berlin, Hamburg, Rotterdam and Amsterdam.
I cannot speak for Paris and London.
Berlin is AWESOME....best marathon in the world in my opinion.
Hamburg is the most underrated marathon in the world. GREAT EVENT. Fast course. Imagine about 1 million SCREAMING GERMANS on the course. Yes, Olympus is now the sponsor. I will be there for my 2:14 in April. I love the race. Course would be a 2;06 if they could afford Khalid or Tergat.
All the Japanese marathons, including Osaka, are organized to a fault. I talked to Lornah Kiplagat (she won Osaka this year) and she says is GREAT. Course is VERY flat. Going westbound from USA to Japan will not jetlag you since it is westbound. You'll be tired for about 1 day max. Arrive one-week early and stay near the river, which has a great running path to train on.
Rotterdam is good, but I had a horrible race there in 2000, and I will probably not be back. Amsterdam is OK, but crowds on the course are very thin when you need the cheers.
The best thing about Berlin is EVERYTHING. The best part of the course is 30-42 km, where the crowds are thickest and loudest, the road is flat and smooth, and it is very negative-splittable.
Jason