I am facing the same question here. My previous employer has Exchange and it worked very well with my 8700g. Later, when I move to my current employer which only supports IMAP, having a BB makes very little sense.
Now I am using a 8525 with WM6. Although the default poutlook doesn't support IDEL which means no push for me, the interface is much better than Flexmail 2007 which does IDEL very nice. Here I am stuck with IMAP every 5 min. Meanwhile, I got a hosted exchange and use activesync to get my personal e-mail as well as contact and celender sync in real time. One bonus is I don't need to pay $10 for BES and another $10 for BB support from the Exchange host. That's a nice $20 saving each month