So, after being given the run-through last week (and oohing and aahing at the NEC N343i, "the gayest phone we've ever seen") I popped into town and picked up a handset today. I'll record my impressions here and update the post over time.
Took handset out of box, plugged in battery and charger. The SIM was already in there. Turned it on, pressed the "I-mode" button. It just worked - I've not had to activate my SIM or anything yet, and I can browse the I-mode portal. That's very nice.
I-mode does indeed seem quick (though services provided by content providers are slightly slower than the I-mode menu itself - which is to be expected I suppose). I'd like to know how; I don't buy the cHTML-is-faster-than-WML argument, it's going over the same networks as WAP does. Maybe the lack of a WAP proxy speeds things? Pages with images on them are noticeably slower - maybe the bundling of e-moji icons onto the handset itself lets O2 provide a faster portal experience.
The NME service seems to be full of dodgy ô character encoding rubbish - more QA needed there, and no real excuse given that there's only a few I-mode handsets on the market right now.
There's a rich portfolio of games. The mobile Sudoku provided by G-Mode is pretty average, but playable.
NEC need help with their UI work in Europe. I like the external appearance of the handset but using it to send text messages is painful. There are actually 2 completely different messaging menus - one you get by pressing the lovely envelope button, the other by navigating to "messages".
So overall: I-mode looks lovely and I'm impressed, but I suspect they'll need to roll out a better overall UI for their handset for this to really take off.