Though all in all, I find the stock Android keyboard and the Google Japanese keyboard better. However, I am very impressed with SwiftKey so far, and it does a very good job to combine a full-featured English keyboard with a proper Japanese keyboard.
There are some things it does much better than Google's Japanese keyboard, such as correction suggestions and prediction for words with "n". Google's keyboard won't always offer suggestions after you enter an "n" until you press the key a second time to convert it to ん or hit a vowel convert it to a な etc. I find this annoying to no end. SwiftKey on the other hand never enters a romaji "n" at all; ん comes out after one press, and then な after hitting an a. This means one less tap to enter a word like わかんない. Tapping "n" twice still inputs ん, keeping with existing conventions. Nicely done.
Now, if they implement godan, it will be just about perfect.
Actually, most of my gripes are with the English input. I find Flow input to be much less good at prediction than either Swype or the stock Android keyboard. Often times I'll end up with some totally random word and am forced to explicitly type out the exact word that I want. I rarely have to do that with other keyboards that support swiping to input English. Another annoyance is with long pressing the period (.) key bring up an alternate punctuation mark. Swype is also guilty of this, which is default to inserting... a period. Why would I want to do that? The stock Android keyboard does the correct thing and suggests punctuation that is not a period (like a comma) after a long press.
UPDATE: As pointed out in the comments, the SwiftKey Japanese beta does in fact have a QWERTY option for Japanese input. I'm not sure how I missed it – probably because I was attempting to change the input layout in the section called "layout". So I'll give this keyboard a second look and update this post in a week or so.
Even after finding the setting, it's still not immediately obvious that the layout can be changed because the they are oddly named ひらがな and ローマ字, which are not appropriate ways to describe keyboard layouts. For comparison, Google's Japanese keyboard uses the terms ケイタイ配列 and QWERTY. (With system language set to English, the keypad layout becomes "12 key".)
In my never ending search for an android keyboard that doesn't suck, I finally got around to trying the SwiftKey Japanese beta. I've grown to like google's "godan" input, and I never tire of the reactions I get from the uninitiated when they borrow my phone. I do however tire of the (albeit more streamlined than in the past) three step method of switching languages which is: long press the space bar, scan a list of keyboard names, tap desired keyboard.
Joining the SwiftKey open beta was a hassle. I first tried on my phone but kept getting redirected to everywhere I didn't want to be, including the play store to download tapatalk. The process starts out completely in Japanese before switching over to 100% English. Here's how I got the keyboard.
Create a SwiftKey account
From the SwiftKey Japanese page click アカウントを作成 (Create account), which will take you to a page that is completely in English. After finally getting all the math questions and captchas correct, I waited for a confirmation email that never came.
I tried again with a different email address and finally got a confirmation email after looking up the name of their iOS note taking app. (Seriously? WTF? If any Japanese person continued after hitting a wall of English, I'm pretty sure that they wouldn't get past that.) Success. Account created.
Download the Japanese beta
For the next step, I went back to do it on my phone. If you don't log in first, you'll just get dumped onto the "you don't have permission to view this page" area after tapping ベータ版ダウンロード (download beta version). The beta is kept in the discussion forums.
Honestly, after all the work to get this keyboard installed, I feel let down. First, I find the stock Android keyboard superior for typing English. I don't often use prediction because it actually slows me down on average – I find it faster to just keep tying the word rather than scan the list of suggestions for it. Granted, prediction would become better with time, though.
More importantly, I'm also not really all that impressed with the Japanese keyboard because it only has one layout, keypad. We are no longer bound by a physical alphanumeric keypad for entering Japanese. Yes, flick-style input is a significant evolution of the number pad, but why must we keep this legacy layout? Why not reinvent the layout in a completely new way now that there are no physical design restrains, which is what google did with the godan keyboard.
So after about a week of using the SwiftKey Japanese beta, I went back to doing my long-press-spacebar tango with Google's Japanese IME and the stock Android keyboard.
This is still much better for Japanese input than the crap offered up by Swype because the SwiftKey Japanese beta has a dedicated enter button and a reasonably good dictionary. If I loved this input style, I would use this as my only keyboard. If SwiftKey implemented QWERTY (or even better) godan, then I would use this as my only keyboard.