My plan is to move to NBI when I move to Snow Leopard. I use my Mini9 every day for a variety of crucial activities (such as maintaining my Calendar, checking e-mail, etc). I don't want to jeopardise that for a gain that would be "intellectual" for me. I run a risk of degrading my Mini9 OSX experience for no real gain, apart from the knowledge that I'm at the cutting edge.
For various reasons, I haven't moved any of my Macs to Snow Leopard yet, so I don't feel a pressing need to move my Mini9 to it either. However, when I start moving to Snow Leopard then I will definitely make the move from DellEFI to NBI.