Apple added support for Japanese in iOS 5.1, but the roll out isn't going as smoothly as expected. According to a Kotaku report, Apple's voice assistant can apparently only handle simple and universal Japanese phrases. When it comes to complex phrases, Siri stumbles while its competitor Shabette Concier performs flawlessly.
Shabette, a recently-launched voice service from wireless carrier NTT DoCoMo, is targeted specifically for Japanese customers and supports maps and local content. It is supposedly faster and less rigid than Siri. You can check out the Japanese video below and head to Kotaku for a phrase-by-phrase analysis of Siri's performance against Shabette.