Pretty damn good. Vodafone Japan’s market share of mobile subscribers had crumbled below 17% in 2006 when SoftBank (SFTBY) swooped it up. This is what SoftBank’s share of Japan’s new mobile subscribers looked like in fiscal 2010 and 2011:

Source: SoftBank

The point here is that not only did SoftBank manage to grab a lead in signing new mobile subscribers back in 2007 — right after it had acquired the ailing Vodafone Japan — it also managed to maintain that lead years later, all the way through 2010 and 2011.

So you could argue that SoftBank’s ability to smack down bigger rivals like NTT-DoCoMo and KDDI did not hinge on the one-time surprise attack it staged in 2007. SoftBank has been able to keep its bigger rivals on the defensive through half a decade, introducing a variety of new pricing and marketing strategies.

If SoftBank does acquire Sprint (S) and/or Clearwire (CLWR), the obvious U.S. analogs to NTT-DoCoMo would be AT&T (T) and Verizon (VZ).

Japan is a hard, hard mobile market. In mid-2011, the mobile ARPU was declining at a hair-raising -5.9% pace industry-wide. Yet as NTT-DoCoMo’s ARPU fell by -4.4% in the second quarter that year, SoftBank’s ARPU rose by 1.9%. In 2011, SoftBank had $6 lower ARPU than the industry-wide average of $58 in Japan. So SoftBank is used to play the scrappy underdog — and it has demonstrated the ability to pull down the ARPU of the leading operator while delivering modest ARPU increase of its own.

Of course, AT&T’s ARPU level is now approaching $65. Its iPhone subscriber ARPU levitates around $100 after the recent moves to eliminate cheap texting and voice buckets.

One could argue that AT&T bears more than a passing resemblance to the vain and plump NTT-DoCoMo back in 2006.

After launching mobile game company SpringToys tragically early in 2000, Tero Kuittinen spent eight years doing equity research at firms including Alliance Capital and Opstock. He is currently a Managing Director at Magid Associates, an Advisor for Next Games and a Strategist for Primesmith, a Finnish 3D imaging and printing app pioneer. He has contributed to TheStreet.com, Forbes and Business 2.0 Magazine in addition to BGR.