The NR in ACR is sensitive to ISO; my default is set to Luminance=20, Luminance Detail=50, Color=25, Color Detail=50. These settings have little or no effect upon low ISO images, but kick in some NR as ISO rises, with good results up to ISO 800 on my D300.
One can make the ACR defaults for a particular camera ISO sensitive, settable in the preferences. I haven't tried this, don't know how many ISO points are required and whether or not there is a rounding function. If this function is fairly "intelligent" one could create a half dozen or so NR defaults and ACR would apply the ones with an ISO nearest the actual image ISO.
All this being said, for more severe noise situations a third party Photoshoop plug-in is desirable, such as Noise Ninja, Denoise, etc. And as far as I can tell there is no disadvantage to using after the raw conversion, assuming all NR and sharpening is turned off in ACR.
I too have been using NeatImage as an export routine from Adobe Lightroom (which uses ACR 6.1) and have been impressed with the new ACR Noise Reduction thusfar. With respect to Lightroom's interface into ACR, when importing images from the camera, I have set up ACR to look at the camera and the ISO to determine the level of NR to apply and it works well. However, you do have to set up the defaults for every ISO level you camera can produce or it will default to Adobe Defaults I believe. It would have been nice if ACR could allow you to define ranges of ISO for applying defaults but as far as I can tell it is only absolute levels.
My impression is that NR in ACR 6.1 has a definite advantage over NR plug-ins in that it seems to do NR at an earlier stage in the processing pipeline, probably before any capture sharpening, clarity, etc.
I agree; however I do find myself "tweaking" the sliders a bit more than I want to in ACR whereby in NeatImage, once the profiles are properly loaded, seemed to work without intervention. I think it is a matter of defining better defaults, including some tweaking to the capture sharpening sliders, which I will do in fairly short order.
Or should I group the images based on ISO, and process differently, like "Low NR" for ISO 100-200, "Medium NR" for ISO 400-800, and "High NR" for ISO's over 1600?
This is probably a better approach, especially for Lightroom users. Instead of stetting NR levels by ISO, just set up a general level (15 - 20 seems to be the right area) for all images imported for a specific camera, set other defaults as well like calibration, lens correction, etc. Than after import, use existing "Smart Collections" that sort by ISO to apply preset NR adjustements to the images as a group. That way if you miss a particular ISO level when setting up defaults you will not have to worry about getting Adobe Default Settings applied to an image.
Just food for thought.