I used Japanese layout keyboards exclusively for a part of my life, and I'd warn you against taking the plunge with an expensive purchase.. there are plenty of cheapo japanese layout keyboards to find you should try it out. There are several major annoyances if you type mostly english, for example:
" is shift+2
' is shirt+ 6 or 7 i forget
The return key is a short L, and usually the backspace is the size of a single letter on a US keyboard, not the double size that you find on a standard 101 layout.
There are some other oddities likes you can type @ or * without a shift. I had a programming job for a summer in Japan, and when I got used to the layout, it actually made some programming stuff easier (especially the arithmetic syntax), but having ' be so far away was still annoying.
Oh yea, and ( and ) are offset by one.
If you plug a Japanese keyboard into a US machine, you can always get it to work. If you use Windows, and you have your keyboard in your device manager set to a US keyboard, then you'll start typing letters that don't correspond to what's labeled on the keys. This is the different layout. Interestingly, though, it's often the case that the computer will end up with the letter that would be in the same physical position as if the keyboard were a US layout (hopefully that makes sense). It means that the same physical positions usually send the same key codes, just that when you have a Japanese keyboard, you have more keys, so you can send a larger variety of keycodes, and then the OS-side has to interpret them correctly to match what is labeled on the keys.
Anyways, long story short, I found that because I use english keyboards most of the time, it was annoying to just have my personal keyboard be Japanese. Granted, that was also when I was in school, and these days I only intereact with keyboards at work and at home, but still, it wasn't worth the trouble for me. It's also why I can't use the Kinesis..