Apple added Japanese support for Siri in its most recent software update, iOS 5.1, but it appears that the voice-activated assistant doesn't perform as well as competing service Shabette Concier from NTT Docomo. YouTube user ysm7651 pits the two head-to-head (video below) and finds that Siri has a couple of important shortcomings. Noticeably, its language processing doesn't seem to handle colloquial speech as well as Shabette Concier and there isn't support for maps or location lookup for restaurants and hospitals yet. It also appears that Siri takes more time to process requests than its competitor, although this could have more to do with differences in the two networks than in the underlying software.
In our experience, while Siri offers most of the same functionality in Japanese as it does in English, there are definitely some areas for improvement. Reading and creating messages, looking up the weather, and performing web searches all work roughly as well in both languages, but we did run into some bugginess when we tried to move calendar appointments around (Siri refused to process a grammatically correct response to a prompt). The service sometimes had some notable issues processing our Japanese, although this is likely due in part to the combination of our English accents and the relative frequency of homophones in the language. The point has also been made that Shabette Concier is much better at finding recipes than Siri, which is due to its integration with Japan's Cookpad recipe service. A particular omission is Siri's inability to tap into Wolfram Alpha in Japanese, which means the service is limited to a simple web search, even for questions about math and science. To be fair, Shabette Concier can't access Wolfram Alpha either, though — it's limited to Wikipedia searches.
Right now it seems that the home-grown Shabette Concier has several advantages over Siri in Japan, but it's hard to imagine that this will slow the iPhone's huge momentum in the country. Four of the top ten phones sold in January were different flavors of the iPhone 4S, and the total sales share across all of them was almost double that of the Arrows X LTE from Fujitsu, the country's top-selling handset that month. In any case, we're interested to see how quickly Apple invests in its Japanese Siri service, and are looking forward to closer feature parity with its English offering over time.