After going through all my statements from the past 12 months, I calculated what would have been the cost for voice service without a Docomo family plan. I used the pricing from two different Docomo plans and the B-mobile Talking FAIR plan. But first, here is the actual number of minutes I used each month, which is greatly influenced by being able to make free calls to family members.
The three horizontal dashed lines indicate the number of included minutes for the Plan SS, the Talking FAIR and the Plan S (25, 32.5, and 55, respectively), assuming only normally-charged domestic calls. The blue area represents minutes used in the family plan.
The Talking FAIR SIM, while not the absolute least expensive, is the best value of the plans with a one year contract. The Value S Plan is cheaper with a two year contract, but it automatically renews every 24 months. Canceling at any point except for increments of 24 months after starting service (e.g., 24th, 48th, 72th month) will incur a ~ ¥10,000 early termination fee. The FAIR's 1 year contract does not automatically renew, so you can cancel it whenever you want, after the initial 12 months, just like in most other places around the world.
What really makes the FAIR SIM compelling is data, especially if you are like me and are 1) have access to wifi most of the day and 2) have an overseas phone. The Talking FAIR may still be a good deal, even if you satisfy only one of these two conditions because of the lack of a renewing contract and no forced tethering charges for any non-docomo branded phone.
I understand that this is a bit of an unfair comparison because of the ¥4,200 yen tethering fee from Docomo, but this is the cost of using a Nexus (or iPhone) on Docomo versus B-mobile.
The FAIR is much less expensive, even if you to add a data charge every month. If a charge only lasts two months, the FAIR is still cheaper, even without Docomo's forced tethering fee. Based on how I actually use my phone, I would expect to pay on average half of what it would cost to use my Docomo branded (and aging) ht-03a on Docomo (~ ¥8,000), versus my much better Nexus One on B-mobile (~¥4,200).
However, I didn't need to look though a year's worth of Docomo invoices to know that the b-mobile Talking FAIR SIM wouldn't be a good deal for a family, since we are eligible for free calling between family members. Over the past 12 months, we used a combined 2,652 total minutes, for which we incurred ¥43,349 in voice related charges. This is an effective rate of about ¥16/minute.
All the calls in blue were essentially free. I could show you the results of a number of calculations, based on a number of differing assumptions regarding usage, but it would be pointless because in all cases the answer is the same. We'd have to significantly alter our usage if we canceled the Docomo family plan. It isn't even economical for me to cancel only my service with Docomo.
So, if you're in Japan for a year or two, perhaps even three, it is seriously in your best interest to understand how much data you tend to consume and consider the Talking FAIR SIM with prepaid data and postpaid voice. But if you have family members, then it is unlikely to be a good value.