i tested the 16-35 f/4 VR lens on the D800E last weekend. It is sharp all the way through the range. Quite impressed by its performance. I suppose it is easier to engineer a higher performing lens with a slower aperture (like how Leica's new crazy 50mm APO-Summicron f/2 costs USD 7200) than with a regular 2.8. It also accepts filters, but the distortion may be unsettling to some, especially for architecture stuff.
Personally speaking, I cannot really shoot beyond 21mm, it seems too wide for me, so that's why I took myself off the pre-order for the Zeiss 15/2.8. As great as that lens promises to be, I just don't use it. So far, I think that the Leica 21/3.4 Super-Elmar is the highest performing 21mm lens I've ever used - brilliant contrast, color saturation, sharpness to the corners, nearly zero distortion (I kid you not!) and performs its best half a stop down at f/4.