I have been a Mac user since the late '80s. From the start, Mac OSs have been very 2-byte character language friendly, unlike Windows. This is one of the reasons that I switched to Macs after formerly being an 'IBM-compatible-PC' user. This 2-byte character friendliness has not changed one bit in OS X of course. However, individual applications handle the issue a bit differently.
I write in Latin characters (English and so on) most of the time, but when I write in Japanese I want to not have to go through extra hoops. I want to concentrate on writing. Here are how some notes on how various text editors handle this issue. My app of choice is at the very end!
One must-have for me for Japanese input is that it has to be inline, within the document. I hate it when the Japanese input pops up in a separate window or popup thingie because it distracts me from my writing. It is inevitable for the conversion (henkan, 変換) popup to appear, but the actual input is another question.
Good old TextEdit handles Japanese input online quite well. If you don't want to spend any extra money or so, and most of your writing is done offline, this is your best option.
BBEdit and BBEdit Lite both handle Japanese input similarly to TextEdit.
The disadvantage of both TextEdit and BBEdit is that I have to set the Japanese font face and size every time I open a document, since my default font in both apps is Monaco.
In and of itself Google Notebook has nothing special when it comes to Japanese input. However, Firefox handles Japanese input within web forms quite well. It keeps my font preference for Japanese, ヒラギノ明朝体, 12pt so I don't have to switch anything. So, you could use Google Notebook fairly effectively. The disadvantage is that Google Notebook does not work on Safari, and your docs are all online, which could be an issue if you are wary about such things. And it's free, as in Google free.
TextMate is a very popular and, let's face it, 'hip' text editor for the Mac. However fails miserably on all counts when it comes to Japanese or 2-byte character input (though it seems that 2.0, which will be Leopard only, will be CJK input-able . We'll see how it fares when 2.0 comes out, but in the meantime, TextMate on its own doesn't even accept Japanese input, and displays it horribly. It's a shame really, since TextMate
is used to be my favorite offline editor for writing blog articles and such. (My favorite programming editor continues to be BBEdit, but that's offtopic.)
If you are determined to use TextMate, there is a plugin that solves the problem sort of half way. It does allow for Japanese input, but the input window floats on top of the actual document, which is a no-go for me. I guess it's useful if you are a programmer who likes to use TextMate and wants to insert comments and so on in Japanese, but that's about it.
Scrivener is a very interesting writing app, geared towards people who write structured documents like scripts and novels. It's great for a number of reasons, but as far as the subject of this post goes, it's great for Japanese writing.
It handles Japanese writing perfectly, inline within the document. Better yet, you can set the font as Japanese, in any Japanese font you want on your system, then save that as a document Template (File -> Save As Template). When you start a new Project, you can choose one of your Japanese-input templates as the base and there you go. I find that since I am speaking and reading and writing in non-Japanese 90% of the time, I need to concentrate a bit more when I'm writing in Japanese, so the full-screen mode is perfect for that. It has now become my favorite writing tool, for both Japanese and English/European.