It's usually an easy sell for a Linux server admin to replace Microsoft's Exchange groupware server with a Linux-based open source server, but users can be quick to rebel if the new groupware suite is too different or doesn't meet their needs. Open-Xchange is very attractive from a cost and server administration viewpoint-- will it work for your end users?
Part of the user experience depends on whether you're using a desktop client for email or the Webmail interface, or if you're getting to your mail via a mobile device. To test OX, I tried a bit of each.
The first thing that caught my eye in setting up the Webmail is that you can connect to Gmail. It looks like MailXchange (the 1and1 setup) just accesses mail via Gmail IMAP and doesn't actually store the mail "locally," which is a bit of a letdown. You can get to Gmail from OX, but if you're trying to migrate it doesn't look like it sucks down the mail so you can close the Gmail account. The plus side is that it doesn't count against your mail quota...
Read the rest of Joe Brockmeier's Open-Xchange review at Linux Planet