If it's anything like the previous-generation BlackBerries, it's shockingly bad. We bought one for my wife on the strength of it having a physical keyboard, and waded through all the hand-over-your-password BIS nonsense. And, well... I guess it *might* work if you never ever want to look at your mail from anything other than your BB. Once the BB has decided what *its* view of your mailboxes is, good luck in having anything else you do via all your other (IMAP, webmail, whatever) clients have any relationship whatsoever to what you see or do on the BB.
Hello RIM? That's the *whole* *fucking* *point* of IMAP - the mail stays on the server, and I can get the same view of it from anywhere, not go through all the hoops we used to have to jump through to fake synchronisation on POP3 clients.
I've since disabled (or deconfigured, or otherwise turned off) the whole BB mail piece, and installed LogicMail, which I heartily recommend. It's a regular IMAP client, it makes IP connections to the mail server, and it all works Just Fine. If she leaves it running, it gets new mail notifications via IDLE. If she closes it, she doesn't get notifications, but it doesn't suck juice or network usage IDLEing. Her choice.