For JPEG images on the default settings, you can start to see a bit of softness from noise suppression, though no actual noise, at ISO 800. (It's still relatively sharp, but soft in comparison.) Below that, it's as sharp and clear as you'd expect from a camera in its class. (Shot with 15-85mm lens.)
JPEGs up to ISO 1,600 remain relatively sharp and noise-free. The degradation jump occurs between ISO 1,600 and ISO 3,200; ISO 3,200 looks visibly soft with the beginnings of noise visible on small details. ISO 6,400 shots display quite a few artifacts. In general, the 7D's noise profile is pretty typical for a camera in its price class.
With low-detail subjects, the 7D's standard noise reduction at middle ISO sensitivities looks pretty good. You can see some noise when zoomed in to 100 percent, but overall it retains saturation and the noise is relatively unnoticeable even when printed at 13x19. As you'd expect, though, just like the D300s's ISO 3,200 images it won't stand up well to postprocessing. (1/80 sec, f5.0, ISO 3,200, evaluative metering, AWB, 15-85mm lens at 50mm, Neutral picture style)
This gives you some idea of the processing trade-offs at the 7D's highest ISO sensitivity. While ISO 12,800 is quite noisy, it's also still quite usable, as long as you're viewing at about 50 percent, printing at small sizes, or don't make any adjustments--in other words, fine for emergencies. (JPEG noise reduction at default setting, Adobe Camera raw processed at default settings. 1/15 sec, f16.0, ISO 12,800, evaluative metering, AWB, 15-85mm lens at 70mm, Standard picture style)
At ISO 1,600, the D300s and the 7D deliver roughly comparable noise results, though I think the D300s' exposure is a bit better. Neither one of them really handles the artificial reds very well, though. (both: raw at 1/15 sec, f5.0, 24-70mm lens at 70mm, color corrected and noise reduction at default settings--visually inspected to confirm optimal results. 7D image scaled to down to compensate for resolution difference.)
This is where the D300s and 7D diverge. While both cameras produce relatively noisy results at ISO 3,200, I think Canon's looks far better than Nikon's. (both: raw at 1/30 sec, f5.0, 24-70mm lens at 70mm, color corrected and noise reduction at default settings--visually inspected to confirm reasonable results. 7D image scaled to down to compensate for resolution difference.)
For panning shots like this, usually the AF system will lock onto the gravel if I inadvertently move off the dog's body. The 7D's AF Point Expansion setting seemed to significantly reduced accidents like that.
(1/400 sec, f5.6, AWB, evaluative metering, 15-85mm lens at 78mm, Standard picture style, AI Servo AF mode with AF Point Expansion point select)