Hello

I think you will find the phones themselves are pretty similar to wherever you live..

We have Sharp, NEC and iPhone and so on..
Probably the biggest difference is the services that are available here.. internet access, GPS and so on are pretty much latest and greatest.

As for International Roaming.. I think most of the companies offer it and it depends on which handset you have whether you can use it or not..

If you were to specify it when you bought one you should be OK.
You would need a Japanese bank account though, because I am pretty sure you couldn't get IR with a pre-paid phone