I installed and got working OpenXchange only to find that they stopped selling the Outlook connector in single or small batches...
You can now only buy outlook connectors in blocks of 25, which costs several hundred £’s.
Your best bet is going to be Horde 3.1 which will be outlook syncable.
It will use an opensource outlook connector, and the SME Developers said that they would be working on an upgrade after 7 went final, so we should hopefully hear something soon.
I agree with you that SME needs a decent groupware option. Unfortunately Outlook is ubiquitous, and once a luser gets used to it there is no way you are going to pry it away from them.
Outlooks implementation of IMAP is terrible, so you can't even realistically use it as an email client with SME.
I eagerly await the horde upgrade!