Blackberry is the only one* with "real" push email; the others pretty much just use IMAP IDLE, which comes very close, but isn't quite "push" the way Blackberry does it. IMAP IDLE requires a constant open connection; Blackberry uses a patented solution which involves the server "waking up" the client when new mail's available. Blackberry also uses compression, which can be useful if you don't have a great data plan.
*There are some other third-party solutions that offer server push (though, again, not quite the way Blackberry does it), including Nokia's own email service. In most, if not all, cases though you'll have to pay separately for this. Nokia's service is free right now (and may continue to be for some users), as is the service and client from Seven. There've been mixed reviews about Nokia's email service, but so far it's been great for me.
If you haven't tried a Nokia E71, drop by a Roger's kiosk and check it out. Much better looking than pictures of it would suggest. Very nice phone - definitely worth a good PSE session.
