We've used it, it's excellent.
We have a few customers that we moved to the SuSE Linux Open Exchange server package and it's a really good product. stable, fast, and relatively cheap. PLus it's much less resource intensive, so it's easy to move onto existing equipment. For example, one customer was going to upgrade their MS Exchange hardware becaue usage growth had led to performance issues. We convinced them to keep the hardware, and move to SUSE/OpenExchange. Not only did performance improve, but we were also able to add all the users from their other plant onto it as well, and still had plenty of overhead on the server. They were also able to stop wasting a salary on a full time MCSE to maintain the MS EXchange server which had required a full time administrator.
