Geez, a lot of you are getting stuck with Exchange. I disagree with a couple of posts in this thread that hint (or declare) that somehow Exchange is more solid than GroupWise. That gave me a chuckle.
As most of the posts seem to indicate, the migration is a pain in the arse unless you've already established (and synchronized) your AD forest with NDS. Even then, it can be problematic, and will surely end up costing the company that does this downgrade (that's for the guy that called the migration an "upgrade."

a chunk of cash, up front and ongoing. I managed a 100-user, two site GroupWise installation -alone- for about 8 years (moved from 5.2 to 5.5 to 6) and had very few issues - most of which were environmental as opposed to bugs. This was while managing the rest of the network, plus all mainframe operations, also by myself. Previously, I had a 25-user cc:Mail setup that needed five times the maintenance & support as GroupWise did for four times the users. Needless to say, I was quite happy with it, thank you very much, as were all the end users. When they were forced to switch to Notes, they griped about how much easier GroupWise is to use. The only people I've heard asking for Outlook is those that use it at home, or came from an MS-only environment, and after they find how intuitive the GroupWise client is, they stop whining for Outlook.
But stability, usability, utility, ease-of-management, low-maintenance and all the other "intangibles" that could be quantified in hard-dollars if anyone took the time, don't balance against the "will of the CxO" in this day and age. No wonder companies are losing money left and right. The bottom line only counts when you have to talk your CxO into adding to staff, or not cutting staff, because people aren't a resource any more, just another expense to cut. Buying lots of hardware and software give them that feeling of getting something for their money, because they can touch it.
I guess it's a good sign for the world economy that so many companies have so much time and money to burn. "Damn the torpedoes! Full speed ahead!"
It would make more sense to switch to an open-source email/calendar/schedule package - less hardware & software investment, with probably just as many techs needed to support it as the MS solution...