I cannot help but think that RIM really missed the boat with IMAP4, yes it is not as prevalent as Exchange (which BES handles nicely), but it is used by businesses as well as consumers.
My employer has been using IMAP4 for many years now and we are very happy with its performance. We have evaluated Exchange over the years, if for not other reason than to give our mobile employees an email solution that works out in the field on a handheld device.
It seems that Palm's WebOS is the first OS in a long time that supports IMAP4 fully. We are patiently waiting for Palm to release a GSM version and then we are going to evaluate it as our mobile email solution (as it does not require any further changes server side).
However, we would very much like to utilize a Blackberry solution as their handheld devices are so much easier to type on than almost any other device in creation. I have touched a PRE and the keyboard leaves a lot to be desired.
I cannot believe that RIM has not either added IMAP4 support to their BES suite of products (in which case we would go that route) or provide robust support for IMAP4 through their BIS product.