Tried one for about 20 minutes, D3S with 400mm f/2.8 and it was very, very good, AF was still quite snappy, there was an obvious loss in IQ but very very acceptable and it did seem to have a slight adverse effect on exposure, images were a little darker, possibly underexposing by half a stop but all in all very good with this combo
D3S and Nikon 70-200mm f/2.8, got to say it was poor in all honesty, differing drops in IQ at different focal lengths and AF slowed quite dramatically and was twitchy, underexposure was also noticeable, very soft images at 200mm to soft at 70mm, would have to say it would be rubbish for field sports where AF speed and tracking are a priority.
To conclude, very good on Nikon long primes but poor on zooms