Give it a try...the delay in getting email via IMAP from my work is up to 15 minutes...unless you have some incredibly time-critical issues to deal w/over email (like you're directing someone in open-heart surgery) you should be fine. The email will actually come in very short bursts - if you are getting a number of emails within 2 minute windows you'll get it almost immediately until you don't get any in a 2 minute window again.
As for tasks, calendar, and notes, etc., you are SOL on BIS...if that is a deal-breaker, then other than 3rd party options for wireless sync (which will take some effort on your part to research and set up, and they have their limitations and issues) you are stuck.
One to consider is that a company called BBerrySYnc is working on OTA sync of Calendar, Contacts, Tasks, and Notes (see the thread in the Aftermarket Software forum on OTA Sync for BIS users). It should be released in the next few months, and (I am a beta tester) looks very promising as an option for BIS users.