IMHO, Flex Mail 2007 does not even come close. It sounds good, until you try it. It also is hard ot remove all the reg entries, that are not auto removed during uninstall. It causes device instability, is a resource hog, a still needs a lot of work. Flex Mail and IMAP server does not even come close to Exchange 2007 and WM6.
Maybe for nOOB, but definitely not for a serious mobile work force. Just my opinion. But if you search the web, you will find many people with the same impression.
I just wish Exchange 2007 ran on Windows server 32. It can be very costly and long testing and install to switch an exchange 2007 and install new servers running windows server 64. Not too many people have several servers laying around that they can install Windows Server 64, Set up the cluster(s) and then install the front end and back end server(s) of Exchange 2007. It really builds on 2003 and has many more features and advantages, but cost is also a factor.