The main reason I switched to Softbank just over 3 years ago was because DoCoMo didn’t offer the iPhone and seemingly had no real interest in offering one. I’d been with DoCoMo since I got my first keitai (携帯 mobile phone) in 2001. They had the best coverage — even out in the mountains of Gunma I’d often have a usable signal — and I had no real gripes about pricing or services relative to others.
Vodaphone (later acquired by Softbank) sometimes offered better handsets, and had more international-friendly services, but DoCoMo had better domestic service and much better coverage. Even now, most of my apartment is a dead zone for Softbank’s signal, while my wife’s au iPhone gets 4 dots when standing in the same exact spot, and her previous DoCoMo handset similarly had full signal strength.
Softbank has experienced incredible growth while they were the exclusive carrier for the iPhone, going from a bit player to a close-third in the carrier market, only about 5 million subscribers  behind KDDI’s au. DoCoMo remains the unquestionably dominant player, but they have lost millions of subscribers over the last few years, and much of that loss is attributed to people moving to Softbank so that they could get an iPhone.
Now that iPhones are offered on all three major carriers (KDDI started carrying the iPhone 5 at the end of last year) we’ll see how iPhones do compared to the other smartphones on offer, without any skewing of the market due to availability. I expect DoCoMo to reverse some of its losses, and iPhone prices and promotions for all the carriers to track closely as they jockey to acquire new subscribers and keep old ones.
I might move back to DoCoMo, when my current contract term is up at the end of this coming February, for coverage reasons. The only service-based gripe I have with Softbank is that they they refuse to unlock your iPhone even if you’ve paid it off and still have a service contract with them. I’ve heard DoCoMo is better about that, also, which would be a plus when I have to travel overseas.