I agree with MMM, and I live in Tokyo. You would have a hard time filling up ten days without getting into much less interesting things. Much better to go to Kansai and see the sights there than waste time in Tokyo after seeing all the major sites. Five days is plenty.

I think it depends on how much you like big cities in general. I love big cities and I tend to stick to them while traveling. I could easily spend two weeks in Tokyo with a few day trips to places like Kamakura, Hakone or Nikko.

I have been to Japan five times so far with another trip coming up. While I really liked Osaka and found some parts of Kyoto lovely I always leave Japan feeling I should have spent more time in Tokyo. I can spend two whole days just going to craft, fabric and bead stores and there is so much in Tokyo no matter what you are interested in. I have a huge list of shops and restaurants that I've meant to go to but still haven't found time for.

I hope to do an Osaka/Fukuoka trip in the future but the next trip is all Tokyo.