I have had the the D300S for just over a year, and its brilliant but I was in the market for a new camera, and initially thought I'd get the D600, but with its inferior AF system (centre packed 39 point array vs the 51 point array) and the oil/dust issue I thought I'd look at the D7100.
I bough the D7100 after 'playing' with it in the shop for about 30 minutes, its AF is amazing (a modified & enhanced version of the one in the D300S).in good light there is very little to choose AF speed wise but the D7100 is more responsive (especially in tracking), and general operation. I low light the D7100 is faster, no doubt.
In terms of IQ the lack of an no anti aliasing filter makes a difference the photos are very very sharp, maybe the 24mpx sensor in general is sharp so maybe I'm attributing too much benefit to the lack of the no anti aliasing filter, but the IQ is better than the D300S (even at low iso's) as one climbs the iso's the D7100 easily extents the gap (especially from 1600 iso up). Whats amazing is that if you check on the DPReview site and compare the iso results of the D600 to the D7100 you'll be amazed at how close the D7100 is (one stop worse than the D600, that's it!)
One of the big pluses of the D300S is its build quality, I love its heft, and those that think the D7100 is like the D7000 think again, I had the D7000 and didnt like it (got rid of it after 3 months), and I can tell you that yes the shape and weight are very similar but the D7100 feels 'tighter' more solid etc. its build quality is brilliant (95% as good as the D300S but without the heft) the lack of heft is both good and bad, once you are used to the weight you'll love it, lighter to carry around etc, but it doesn't have brick like feel of the D300S (where you feel you could use the D300S as a self defense weapon and still take a few snaps after).
The D7100 has the same weather proofing as the D300S and the D800, which is great. I don't use the video functionality on my DSLR's so I wont say anything about these aspects, though from what the specs say (and numerous reviews, the D7100 wins hands down in terms of video).
I actually love the 1.3x extra crop option (at 15mpx odd), this option added to one of the Fn buttons adds a nice bit of additional versatility when shooting, I love it.
The only real benefit the D300S has is the buffer size (when shooting RAW, and only at 12 bit, at 14 bit the D7100 holds its own). But if you shoot in fine JPEG for example the D7100 is as good as the D300S.
So yes, I'm impressed, as a package I think there's nothing to beat it, and those waiting for the D400, I really doubt its gonna come, there just isn't room for it, the price difference between the D600 and the D7100 is just too small to accommodate another model without impacting on sales of the other 2...
For those with the D300S and happy with it, its an awesome machine, but those with it that want to upgrade (DX) get the D7100 its awesome and its better than the D600 IMHO ( the extra premium for the D600 aint worth it).