Having migrated from a MS Exchange/BES environment to Zimbra I can say that the current (third party) available BB integration is not where it needs to be. While it does work, the end user experience mentioned above is indeed poor in comparison. The majority of my users who I migrated have quit using it all together.
BES is just that. It's Rimm's software into their cloud. It runs on windows - that's not Zimbra's fault. If you don't like that, don't buy Blackberries. Any other method of using a third party client to talk on a BB will kill the battery life and lead to a much poorer user experience.
Just my $.02, and while I hate dealing with MS products just as much as the next linux user, I can't wait to get this up and running later today so I can dump the current solution.