If you want to stay connected in Japan, there are 3 choices available for you. The first one is activate your data roaming using your own sim card; the second one is buy new prepaid sim card in Japan; and the last one is rent a mobile wifi. The last choice is the cheapest way if you're travelling on budget with group of friends.
When I went to Japan last November, I travelled in group of 6 people and we decided to rent a mobile wifi. I booked the router through Japan Wireless. They offer 4 types of routers with different speeds and coverages. I chose to rent their cheapest choice, pocket wifi by EM Mobile with 21Mbps speed and it can be shared up to 5 devices.
The cost to rent this router for 9 days is 4,700 JPY (incl. tax) and 500 JPY shipping fees will be added. The package consist of one router (the mobile wifi itself), one extra battery, a charger, an instruction to use the router and a big envelope to return everything once the rent period is over.
They offer 2 choices of pick up method, you can collect the package by yourself at the airport or they can send the package to hostel/hotel where you'll stay in Japan. To minimize any errors during shipping process, I decided to pick up the package itself at the post office in Kansai Airport.
Let me sum everything in step-by-step process :)
1) Open Japan Wireless website
2) Choose the type of router that you want to rent
3) Fill the order form
4) Pay the rent fee with credit card
5) Once the payment made, they'll sent a confirmation letter to your e-mail (don't forget to print out the letter)
6) Pick up the router at the airport or reception desk at your hostel/hotel
7) Stay connected anywhere in Japan! :)
8) At the end of your rental period, put the routers (along with the extra battery and its charger) in the envelope that Japan Wireless has provided and drop it into any postbox in Japan.
So simple and easy right?
I didn't face any trouble with the connection while using this router. It's quite steady and the speed was as fast as their promise. But, to outsmart the battery problem I had to turn on-turn off the router all day long because the total use of the router battery and its extra are just around 8 hours. Another obstacle that you'll face if you decided to rent mobile wifi is you have to stay in group with the person who carriers the routers. If you decide to go on your own separate way, you'll lose the connection. You won't face these 2 problems if you use your own sim-card or buy a new prepaid sim data card in Japan. The bottom line is you have to pay extra for more convenience service :)
Japan Wireless also sell prepaid sim data card for 4,250 JPY (plus 500 JPY shipping fee). You can use this card in Japan for 30 days with 1 GB data usage limit. But it's important to be noticed that this sim card is for internet data use only, you can't making or receiving any phone call.
Oh, you have one last option too: rely on free-wifi service in Japan! Limited area though. Well, the choice is yours! :)