oulook as client - I know one firm doing this. They are using the plugin ( no longer supported, but apparently it works)
My issue with this is simple:
1. Outlook runs decently well with an Exchange server on the backend, but run as a client with a local PST file it is a useless piece of garbage, IMHO. I have 2 users running this way at the moment, and it is nothing but trouble. This gets lost, that gets lost, things dissapear, PST gets corrupt, etc. You can't share the PST without going to 3rd party apps (like workgroup share) and backup is a problem unless the user closes out of it.
2. Outlook is slow running against groupwise IME. really slow. I have not run it with the plug in, that probably helps a lot. Native OL against Groupwise is a non-starter IMHO. It takes minutes to launch.
3. You now have opened yourself up to the most vulnerable part of Exchange - the Outlook client. The overwhelming majority of viruses written are written against Outlook. I think it's cheaper (and simpler!) to just paint a big target on your forehead and drive down to a local shooting range.
4 - Don't know, but *highly* doubt you can read your archive or use the backup feature (use restore area I assume?) or any of the groupware functions. Not 100% on this though. As it is not supported by either vendor I think you are almost asking for trouble as any hiccup that comes along is going to be something you own.. is that the path you want to go down?