I'm pretty impressed with the iPhone's choice of input methods for Japanese text -- first, you've got the standard romaji input on QWERTY, which translates roman characters to Japanese. Easy. Very familiar.
But there's also a gestural kana input -- put your finger on [あ] and swipe up to enter う. Pretty cool. Not faster, at least for a native English speaker like myself, but not bad either. A little less error-prone than the direct romaji input, I think.
Finally there's the Chinese scribble pad -- write kanji with your finger and see if it's recognized. I don't have very much luck with this one. Usually I write something, think "that doesn't look anything like it's supposed to", and find the software agrees with me. (And yes, I know, stroke order is important.) There's also the problem that the Japanese simplified characters, the 新字体, aren't quite the same as the Chinese. The Chinese input has no idea what to do with 与, for example. (It can, however, turn 写 into 寫.) Still, though, if you want to look up unfamiliar kanji, the ability to write them directly is fantastic, and it's good to get some practice with the actual character form, rather than the keyboard input.
Truthfully, though, the phone amazes me for its ability to figure out what I mean, especially in a handheld device. The other day I typed in 無愛想 as むあいそう and got the correct kanji. (It's actually read 「ぶあいそう」. My desktop does the same correction, I note.) We've come a long way. It's really quite marvelous to see the device almost understanding what I mean.