We rented one for two weeks in November. It was invaluable on our trip and I would highly recommend it.
Reception was good in most areas of the cities we visited. Transmission speeds did vary a little but were still more than acceptable.
We used it to check hyperdia a lot, this makes planning the next train trip really handy as we did a lot of travelling around many places between Fukuoka and Tokyo. We also used Shin-Osaka as our base and did a lot of day trips (Kyoto, Hikone, Hiroshima, Kobe, etc) where we didn't know what time we would be heading back so it was great to know whether we needed to rush to the station or if we had some extra time up our sleeve. We also used it to check Japan-guide for suggestions on what to do in an area while we were there.
It was also very handy when walking around places to use Google Maps (with the iPhone's GPS), either to work out where exactly we were, or to use it to navigate down the streets to our desired destination.
Back at the hotels at night time it was often easier to use the wifi device to surf the internet, check e-mail, view youtube etc. although sometimes I used the wired LAN connections for the Laptop. I tested the speed (speedtest.net) in most of the hotels and it was more than fine for Skype.
We had three devices connected to the wifi, iPad, iPhone, and a Laptop computer. All connected with no problems.
Although battery life is reasonable and we never ran out of power I was only switching it on when we needed it. Battery life is listed at "3-4 hours". If you think you will need more time between charges then I would recommend you buy an extra power pack that can charge a standard USB device. You can make your own if you are handy with basic electronics and can use two AA batteries or there are a number of commercial devices available for this. The device came with a USB cable and a wall charger so we just charged it whenever we were at the hotel and left it running overnight.
Depending on who you rent from and where you pick it up, the only other point is that it may come in the original packaging which is a box a fair bit larger than the actual device. You may need to carry this box with you (or arrange to leave it somewhere).
We rented from a company called Exsili who were excellent to deal with. They don't have a big English presence but they were efficient, quick to respond, and it was waiting at our hotel in Osaka when we arrived. The hotel kindly helped shipping it back to them at the end of our stay.
http://www.exseli.com/rental/cat/20/entry/1872The only other point worth mentioning is that these devices seem to be in fairly high demand as they are so useful, so once you know your travel dates (or have booked your airfare) then try and make a booking to rent one with one of the companies. I tried on a previous trip about one week out with five different suppliers and they all had none available.
Sorry for the lengthy post, but hopefully that covers everything! Feel free to ask any further questions.
