When they added IMAP I switched to see what the big deal was, and my emails were no longer PUSHED. They would take about 5-10 minutes to show up. I went back to POP, and everything was instant again. They both seem to be able to do two-way reconcile from the device itself, but not from the Gmail app to the BB.
With pop the mail program pulls your messages off the server and downloads them in there entirety.
IMAP only downloads the message headers and when you open the message up it then downloads the rest of the email. Furthermore IMAP will sync things like deleted messages and whether or not a message was read with the server. So if you get an email and your using IMAP. Then you view the message on your BB then you click Reconcile Now it will mark the message as read in gmail so when you log into gmail it will show the message as read.
IMAP is also good for syncing email between multiple computers