Hanasshe,
I lived in Japan for 10 years and I got used to using a Japanese keyboard but make sure you are okay with one before you buy it. Many of the punctuation keys are in different places and even more annoying is they have a shorter spacebar. They also include a few keys that are not needed unless you type Japanese. The real problem comes when you get used to it and then you suddenly have to use a non-Japanese keyboard.
Other than the keyboard, they are pretty much the same but for me, it would be worth it to buy it in the UK or travel to the US or Canada and get one. Normally in Japan, if you are tourist, you don't have to pay sales tax in some shops. In other shops, if you pay it, keep the receipt and you can get the sales tax back at the airport when you leave I believe.