hi,
it depends on what you want to achieve. Open-Xchange meant to be a replacement of MSExchange just as OpenGroupware.org. Both were commercial products and later they opened the code. Both of them have outlook connectors (OX calls it OXtender). OpenXchange is a bit harder to integrate with Samba3 in PDC mode. But they can perform a very good job. You can buy commercial support also. Kolab also has an Outlook plugin to work seamlessly with Kolab but I didn't tested it.
bye, Ago