Seems to me that we are in a polarizing time, with a large set of enterprises moving even more deeply into the arms of the big groupware vendors, Microsoft (with Exchange) and IBM (with Domino). And at the other end of the spectrum gathering market share steam is customized open source, AJAX-fronted, and Web mail-proficient messaging components solutions.
The result? The middle ground between these messaging methodologies is being hollowed out, ala GroupWise.
Don't forget out there, Microsoft support for Exchange 5.5 bites the dust come Jan. 1, so make your strategic messaging decisions fast and furious, oh ye of slow upgrade motivation.