The Bottom Line
Mac OS X Mail 3 and later does support IMAP-IDLE without the IMAP-IDLE add-on.
- IMAP-IDLE for Mail.app adds IMAP IDLE support to Mac OS X Mail
- You get new mail and folder updates as soon as they arrive and happen on the server
- IMAP-IDLE for Mail.app cannot be disabled for individual accounts
- IMAP-IDLE for Mail.app only watches Inboxes and cannot be turned on for other folders
- Growl notifications do not work with mail gotten via IMAP IDLE
- IMAP-IDLE for Mail.app adds support for IMAP IDLE to Mac OS X Mail.
- IDLE allows IMAP servers to push new messages and folder updates to Mail.
- New messages arrive immediately without periodic checking.
- IMAP-IDLE for Mail.app supports Mac OS X 10.4.
Guide Review - IMAP-IDLE for Mail.app 1.06 - Mac OS X Mail Plug-In
With the IMAP-IDLE for Mail.app mailbundle, you get all this goodness with no effort or obtrusion. The plug-in does not come with an installer, but moving the plug-in to Mail's "Bundles" folder (and enabling mailbundles) is really all it takes. Then, provided your IMAP server supports the IDLE command, new messages in the Inbox appear instantly and you can safely set Mail's periodic checks to "manual".
It would be nice if IMAP-IDLE for Mail.app could turn on instant updates for folders other than Inboxes, and maybe let you disable them for certain accounts altogether. On the other hand, IMAP-IDLE for Mail.app's refraining from any options or intervention or grabbing your attention is, of course, a virtue.