I didn't mean to write a book, but...
The staff at the SoftBank counter in Narita speak excellent English. I have never had a problem with them. I can't vouch for other companies.
For starters: Make *sure* your phone uses/can use the 2100 band for data, that it's GSM, and that it's unlocked. A lot of US ATT phones don't use this band, and won't work in Japan. Do your research first. (Unlocked GSM iPhones are generally okay, as are unlocked GSM Nexus phones.)
For a plain old phone, the SoftBank SIM is a decent deal. But data-wise, it all depends on your use pattern. For me, 3 months in Japan made it a non-starter. In 6-7 days, I would have covered the cost of a BMobile 4-month FAIR SIM in data.
But if you're just there short-term, then the SoftBank SIM may be a better deal. Check the SoftBank and BMobile websites, or the blog formerly known as softbanksucks for comparisons.
Here's a link to the B-Mobile posts there:
http://www.japanmobiletech.com/search/label/b-mobile
There's a lot of stuff in there, but the basic takeaway message is that the 4-month Fair and 1-month U300 data plans are still in place for non-4G phones, at the same old prices I had. There are also data plans for 4G phones, but they're more expensive.
BMobile has a ton of different plans, but generally the 1-month U300 (not the Visitor U300!) plan is the best one for most short-term visitors. 30 days of data, 1GB cap, 3G speeds or best effort, 3480 yen. No rollover to the next month if you buy extra months. (Which you can do for the same 3480 yen, IIRC.)
Supposedly you can get the 1-month/1GB U300 at Costco Japan for 2980 yen, saving you 500 yen. Your US Costco card will work there, according to the poster.
Note that BMobile has come out with the U300 "Visitor" version... it's slightly cheaper, but... it doesn't allow VOIP or streaming for some reason, and the clock starts the second day after shipping, whether you get it or not, whether you're in Japan or not. Also, you can't recharge it, unlike the standard U300 and the Fair. To me, the standard U300 looks like a better deal if you're going to use Skype/streaming at all, or if you plan to stay longer than 30 days or want more than 1GB of data.
The Chameleon... meh. Someone may want it. Not me.
I had wifi at home in Okazaki, so I wound up only using 850MB in 3 months, and that's with a lot of Google Maps activity. (Google Maps saved my bacon a number of times.) Pre-loading the maps at home on wifi before I went out saved a lot of data as well. I think they've moved that from labs to the main product in Android. Just be aware that your maps will all be in Japanese.
If you use a Gingerbread phone, note that data-only SIM cards will cut your battery life in half. Not sure if they fixed it in ICS. I don't know what it does to iPhone battery life.
Looks like there's a fix for some phones. Info here:
http://www.japanmobiletech.com/2012/03/f...ttery.html
I did notice that my battery usage on my old Nexus One pretty much doubled, even on standby. Not a huge deal. Just recharge it every day, and if you use it a lot, carry an extra battery or external charger if you're on iPhone.
The Yodobashi folks will probably help you set it up if you need help. I didn't need any help from them, but I've heard stories on the SBS blog that folks who had issues got all kinds of help from them. BMobile also gives you pretty thorough instructions on their website. (It's been 9 months, so I'm a little foggy on that.) If you go to Yodobashi, you will need to speak some Japanese most likely. It definitely greases the wheels. Also, the BMobile website is mostly in Japanese, except for the U300 Visitor stuff... so you'll need some Japanese knowledge for that as well.
Edited: 2012-06-14, 10:07 am