I owned a first-gen iPad and four black iPhones, but with my most recent purchases (Retina iPad and iPhone 4S) I went white. I have no complaints — I'd been a little worried that the white frame would call attention to itself or lessen the “blank slate” feeling of the devices, but it really doesn't. I think blue or red or something would, but white is still so neutral that it isn't an issue. I'd say go with whatever you think looks nice.
One thing: if you are a two-iPhone household, definitely get one in each color. Makes it easy to tell them apart at a glance!
I currently have the iPhone 4 which was only in blank at release so I am happy to be getting the white iPhone 5.
I had a white 3GS but it was hidden by the case. I am contemplating not getting a case for the iPhone 5, I rarely used one for the 4. If I don't get a case I will be getting AppleCare+.
I think black and slate looks better. They just go together better than the white and slate.
Black iPhone 5 FTW
I have always owned black iphones, but i ended up getting the white version this time around, and it looks great! I have it in my hot little hands right now!