I've been using M$ Exchange for years, and finally switched over to an open source groupware server, and after evaluating to competition, OX was certainly the best in terms of functionality, ease of install, and their support is great if you can catch them during business hours EST.
Overall, the project is great and I don't mind paying for the commercial peripherals, support, installer, documenatation, etc. I never realized how I could integrate email with documents so effeciently. When I receive an email with an attachment, I can export it directly into a folder in the Documents store of OX. Then using Netdrive, I can access the OX folder structure from the windows explorer - very cool - can add docs, move docs, and all changes will be reflected on the server.
The Outlook OXtender has gotten 10x times better than it was a year ago, it is almost a true replacement for Exchange with no change to the Outlook users. I strongly suggest it for for SMBs