I've used this adjustment quite a bit and it does wonders for removing noise however, it's not selective at all and I end up with very soft images even when shooting at an ISO as low as 6400. This slider, although a great tool, is not the end-all-be-all of noise removal and has actually left me disappointed more often than not.
I assume this is a typo? 6400 is an extremely high ISO. You would expect it to create very noisy images which lack fine detail. Therefore if you apply noise reduction to them, of course its going to create soft images.
After applying noise reduction to an image, it is possible to apply "proper" sharpening to get some of the detail back without reintroducing the noise.
By this I mean something like Photoshop's Unsharp Mask tool, which lets you apply the sharpening proportionally. Do not try to sharpen your image inside the DAM the sharpening tools in Aperture / Lightroom / Capture are terrible!