I am in a bit of a dilemma for when I upgrade to the Nikon D700 in the very near future (dependant on funds). I will need an ultra wide angle lens (UWA) to replace my Sigma 10-20mm and the Nikon 18-200mm VR or as my friends refer to it as the "lazy" lens. The problem is I can only afford to only replace one of those lenses for the time being.
Although, I generally use the Sigma 10-20mm for about 90% of my shots which is swaying me towards the 14-24mm f/2.8. The issue is if I choose the UWA option is that I will have nothing between 24mm - 50mm and between 50mm - 70mm so I keep vacillating between the 14-24mm f/2.8 and the 24-70mm f/2.8 as a replacement lens.
Having said that the D700 supports DX lenses and I could keep using my Sigma 10-20mm but that defeats the purpose of having a a full frame dSLR. Still leaning towards the 14-24mm at the moment. Looks like I will have to start saving up for it. From all reports it is the sharpest ultra wide angle lens out there.