Used to run with a pair of D3S's and under the truism that there exists no single wundercam for every situation, recently moved to D3S + 800e.
Our D3s will be with us until it implodes -- brilliant all-around camera with amazing balance of speed, resolution, high-iso and acuity, etc. Great file size -- if significant cropping or large prints not required.
I didn't personally see the cost/benefit of upgrading any of the D3S units to D4. For what we wanted/needed compared to what the 4 offered vs the 3S, the math didn't work -- for us.
Still morphing the 800e into the process, but the files are gorgeous, AF solid, etc. Our biggest PITA are the small things we need to adjust to from having two so different bodies. Different batteries, chargers, control layout, feel in hand, AS plates, cards (hate SD), etc. The small stuff.
I'd actually looked around for a clean D3X because of the above and fact I love the pro body size (using w/grip close, but not same), but just couldn't find a clean suitable unit in Canada (15% VAT at border, Nikon Canada service, etc). It got to the point that for the same $$, a new 800e and it's merits eventually outweighed the softer issues that had be initially favouring a used 3X.