It’s definitely worth going one step beyond google’s own instructions on the matter and correctly configuring the drafts, sent and trash folders in both clients:

Mail.app

Choose the google folder for drafts in the sidebar. From the os x menu bar at the top of your screen choose ‘mailbox’ and then ‘use this mailbox for’ and then ‘drafts’. Do the same with the other folders.

iPhone Mail

Having done the basic setup, go to the settings app, choose ‘mail, contacts and calendars’, navigate to your mail account and choose ‘advanced’ and then setup the ‘mailbox behaviours’ as required.

NB if you are a fan of ‘labels’ in gmail, bear in mind that these equate to copies of the email in IMAP folders. That means that if you have, as I did, several thousand labelled emails from a different account you will have more than one copy of each downloaded to mail.app, assuming you chose to download your messages- and who wouldn’t?

The iPhone seems to work pretty flawlessly, but mail.app sometimes seems to take a while to catch up with changes (especially emails you’ve read on the iPhone while mail.app was offline) although it always seems to get there in the end.