I have worked with Japanese for many years and have made a lots of trips to Japan. I have Japanese friends who graduated from the imperial universities and some with kids who are now attending IUs and U.S. schools as well. With out a doubt, most of the imperial universities are excellent schools and are some of the best in the world, but, they are not the best. There are many lists and world rankings for universities and not all of them are U.S. lists and the majority of the respected lists place about 2/3rds of U.S. schools in the top-50. If you are not from the U.S. you probably want believe this, but publications like Newsweek and U.S. News and World Report, etc., go out of their way not to appear biased. Btw, it is not as difficult as you think to objectively evaluate a university, there are certain things they all have in common.
It is a big accomplishment to get accepted to one of the imperial universities, but once you are in, academics are not the priority. Home life for a lot of Japanese kids is so regimented that once they go away to school, it is party time...which is not unlike a lot of U.S. schools as well. To the guy who criticized U.S. secondary education; unless you live in the Amazon Jungle or Central Africa, I bet you can't make through a day w/o using, touching or somehow interacting with a U.S. invention. (Give it a try.) Yes, the U.S. hires foreign professors, science and technology workers to feel a workforce gap, but at top end, like space exploration, stem cell biomedical research, nuclear physics, etc., it almost exclusively U.S. brain power.