My workplace (a k-12 school district) looked at this as an alternative to Exchange because it was cheaper (in both licensing and IT staff) and provided the functions that we wanted (callendering,todo lists,e-mail); except that it would not sync well with our Palm Tungsten E2s that we standardized on for our administrators. We paid for the version that let us sync to Palms. They provided up with support, and it still didn't work.
The sole reason we are not using it is because it doesn't work with the palm hotsync utility. So, we currently have Twig, which is okay, and it looks like we will be sticking with that for a while.Also, Zimbra is clunky and slow on old machines.