Imho
I've owned 7 or 8 cases now for my iphone and thought I'd weigh in.
I think that my overall favorites right now would have to be the crystal film in conjunction with the Incase Slider or crystal film in conjunction with the Griffin Wave ( I buy cases like they are going out of style).
Each has a really nice design aesthetic that stands out without being over-the-top.
When I purchase the new iPhone 3G, I'll buy whichever refreshed version of the Wave or Slider is available. I'd give the nod to the polished black Incase slider for the black iPhone 3G only because the look of the finish is so similar between device and case. I'll most certainly pick up the film as well.
Cases i've owned (all used with crystal film):
Some original Belkin flip case that I returned immediately
Power Support Crystal Jacket (meh design)
Incase rubber case (stretched out)
Refreshed Incase rubber case that didn't stretch (got tired of)
iSkin Revo (great for working out...too bulky)
Agent 18 Eco Shield (nice and slim, unremarkable)
Griffin Wave (stylish, small footprint)
Incase Slider (stylish, has a bit of an "it" quality)