Tried this out yesterday using a bogus Gmail account (using IMAP).
Seems to work fine, however mail receipt is not instant, takes about 5 mins after sending mail to receive it.
I can't seem to get red LED notification, but I do get the envelope coming up.
At the end of messages sent from my BB, I get "Powered by Shangmail" as the signature.
Very tempted by the service, as it is free (however, how long will it stay free). The other thing is that messages will pass through (and stay?) on their servers in China (co. seems to be located in Beijing). Not 100% confident with that (but then again who do you trust more, the Chinese government or Gmail, or at least which is the lesser of two evils?)
I'm presently using the BB Gmail app, which I am generally quite happy with, but being able to integrate and utilise the email function of the blackberry is very tempting.