There is a separate emoji panel, but in my experience you tend to remember the name of the emoji used the most, and in 10-key mode it's pretty fast to type a 2 or 3 kana word.
About the android phones, I was a bit surprised by the low quality of Japanese support. But I didn't try the ATOK keyboard nor the input available on Sony phones for instance, only the Google IME. To Apple's credit, they've been doing IME on the mac for long enough to have a very good integration of the emoji in the system dictionary.
But really, my point was not only on the colourful emoji, but more on the fact that characters like ↑,♡,◎,△ are just unusable on a daily basis on a US/european computer, but are super easily summoned on a system with an IME. And Japanese people have been using these kind of pictograms in their communication for decades, the emoji used on the phones being just an advanced kind of the same thing. I think that would be the main cultural difference regarding to emoji.
The Japanese android vendor keyboards do seem to be generally much more polished than google's effort, although the latter is ok for basic Japanese input.
The main issue with conversion-based emoji seems to be discoverability.... Besides those reflecting basic concepts, there are tons of odd or intricate emoji that you'd never even imagine existed unless you saw them first; even once you know they exist, figuring out what input maps to them is at best a crap shoot (I've never found any way of getting the input system to tell you)....
About discoverability, when you start being a heavy user, the next step is to have custom entries in the user dictionary. It's almost mandatory for 2chan style smileys[0] with 7 or 8 characters in a row, and it also helps for obscure characters, emoji or not (there was a time "girl moji"[1] was a thing, with every letter splitted into 2 or 3 parts. I can't imagine that thing working without having all the letters in a user dictionary).
I personally find Google's offering nearly unusable as someone who primarily types Japanese via romaji. The vendors do a much better job but I found the fastest way for me was Swype. Emoji support is basically non existent though
About the android phones, I was a bit surprised by the low quality of Japanese support. But I didn't try the ATOK keyboard nor the input available on Sony phones for instance, only the Google IME. To Apple's credit, they've been doing IME on the mac for long enough to have a very good integration of the emoji in the system dictionary.
But really, my point was not only on the colourful emoji, but more on the fact that characters like ↑,♡,◎,△ are just unusable on a daily basis on a US/european computer, but are super easily summoned on a system with an IME. And Japanese people have been using these kind of pictograms in their communication for decades, the emoji used on the phones being just an advanced kind of the same thing. I think that would be the main cultural difference regarding to emoji.