I have rented a wireless mobile router for all my three trips to Japan. My favourite Japanese mobile router company is Japan Wireless because I find its charges comparatively cheaper than the other telcos.
I have used Japan Wireless on my first trip to Japan and was very satisfied with its services. On my second and third trips to Japan I still ended up using Japan Wireless because it still offered the best rate when compared to other telco.
The entire process of picking up and returning the mobile router was a bliss. I could pre-arrange to pick up the mobile router at the Airport, any Post Offices or at the front counter of my hotel. When returning the mobile router I just dropped them at the Post Office Box at the Airport before flying out.
What I also liked about this company was its communication with its clients. It emailed to tell me that it had sent out the mobile router to our agreed location of pickup two days before the start date of the rental. Similarly after it received the returned mobile router at the end of my trip it would also drop me an email, so no nasty surprises.