This is really good news because now you can sync your mail, your calendar and your contacts with your iPhone. As I was already using Google Sync for contacts and calendars, setting up GMail push was really easy: just the flip of a switch!
If you haven't set up your Google Sync account on your iPhone, just follow the instructions on the Google Sync web site.
As far as I can tell, mail is pushed to the iPhone almost instantaneously. Nonetheless, there's a thing I'm not really happy about. I miss is a notification popup: no one is ever shown and the counter on the mail icon is the only information you're given when a mail is pushed:
I would expect a mail to be managed just like an SMS or even a phone call: checking periodically sort of defies the purpose of a push notification...