Over the past 60 days I have spent nearly 30 days in Japan with a large amount of that time being mobile. In order to stay connected at all times I rented a wireless card on each trip.
The first Wireless card a FOMA A2502 by DoCoMo failed to work on my PC. I spent nearly 3 hours one day in a main DoCoMo store having them trying to get it to work. Essentially Epic Fail had something to do with American Version of Windows Vista attempting to run some Japanese drivers.
The second wireless card I tried was from EMOBILE and it worked great when I was in a coverage area. I found that so long as I was in a MAJOR city that it would work but anytime I was in smaller communities there was no coverage.
The highest speed I got out of the service was 756k down and 256k up. This is considerably slower than service in the United States. For a country where everyone has a cell phone with an amazing number of services and features the mobile broadband service here was very disappointing.
In the near future I will not rent a mobile broadband card as it was simply not worth it.