nag 57,
Our Company recently jumped from NX5 to 7.5 and the process has been relatively painless. 7.5 seems to run a little smoother and has a lot more tools for direct modeling. I think the hardest adjustment has been just learning where some of the commands from 5 have been moved to. They've left the command finder in place though, so its really not much of an issue at all. As John as said, the refile utility is a great idea if you make the shift, as assembly mating conditions have undergone a major overhaul functionality-wise it seems.
Hope this helps, but I'd recommend making the shift for sure.