I'm always a little leery about doing something that's not part of the core of SME, but I'd really love to get a good tie-in between Thunderbird, Lightning and Kronolith.
The shared calendar feature of Exchange/Outlook was one of the only things that made me hesitate when choosing this platform for this latest server. In the end, I couldn't rationalize $15k in licensing for a shared calendar, so if I could get this to work, we'd all be tickled pink.
My users used to use Kronolith via webmail, but complained because it wasn't integrated into their mail client and forced them to have an additional application running (oh, the horror!) This last point is why I'd be leaning towards using Lightning instead of Sunbird.