Hi Rob,
Since I am stuck between a rock and a hard place and need to sell a couple of lenses to fund any new lens purchase, my workhorse lens for my FX camera is my 10 year old 17-55mm F2.8 DX lens. The lens's quality is not in question and I have no issues using it on an FX body. I have one of the front buttons on the camera programmed to switch between FX and DX crop so I can quickly switch between the two modes without dropping the camera from my eye and I can very happily use the 17-55 DX lens in FX mode from around 28mm up to 55 with no vignetting making use of the full FX sensor with my DX lens.
I also still use my 10.5mm F2.8 DX Fisheye on my FX body as well with no issues.
I would really like to offload my 17-55 F2.8 DX and my Sigma 100-300 F4 (with x1.4 tele) to part-fund a 24-70mm F2.8 but until then, I am happy enough with my DX lens on my FX body since the image quality from this lens is outstanding anyway.
One other unintentional "benefit" of using DX lenses on the D600/D610, No matter what other people say, there is still an issue with dirt/oil spots on the sensor and on my D610 it is mainly in the bottom right hand corner (top left of an image) and using the DX lens removes this problem since that part of the sensor is not covered by the DX lens when used in DX crop mode ;-)
My sensor was cleaned by AJ Johnstone in Glasgow earlier this year before we went to Norway and it is as dirty as ever and it absolutely looks like oil/moisture marks rather than dirt marks.
Ross