We are moving to 6.0 by Christmas, and would also love any insight on the transition anyone has to offer.
Michelle Lavoiehave you tried setting up a 30-day trial of the platform? We used one here with a small user group (I was a part of it) and it really helped to get to alleviate any fears. The new platform is, IMO, heads and shoulders above 5.x.
For on-premise JIve a significant architectural change is to Jive Search. Jive content search can now be Cloud or On-premise. On-premise requires additional search nodes to be installed.