Quite a few of my shots are done at ISO 1600 and above so noise reduction in post processing is something I pay attention to. I've been using Darktable over the last year and just wanted to do a quick comparison to Lightroom 5's NR features. No I haven't bought Lightroom 6 yet, I barely use it as is so sinking more money into it seems like a poor decision.
First, comparing a couple of well lit shots at ISO 1000 from the X100s.
Image with no noise reduction
Darktable profiled noise reduction, settings in image
Lightroom 5 noise reduction, settings in image
There isn't a lot of noise at ISO 1000 so either program really does an OK job. Darktable seems to preserve more details but at the cost of some muddy colors and artifacts in places. Lightroom's output is smoother. I'm not sure this is the noise reduction or just differences in RAW processing and camera profiles, the screen captures exaggerate it some as well.
Next some rather dark photos from the Dark Sky Observatory. X100s at ISO 3200. Darktable's profiled noise reduction takes a fair bit more tweaking to get satisfactory results. I've taken the approach advised by Robert Hutton and done two passes of the module. The first set to wavelets mode, blending uniformly for HSV color to remove the color noise. This Darktable handles quite well. Where it runs into problems (like with the other images) is luminance noise.
ISO 3200 no noise reduction
Darktable color only noise reduction
Darktable color and luminance noise reduction
Lightroom noise reduction
After comparing the darker shots you can see where Lightroom pulls out in front with luminance smoothing. More detail is lost but the image is generally better IMO. Darktbale's profiled denoise on a HSV lightness layer produces very blocky results, it kind of reminds me of Noise Ninja's output from 2007 or so. It's especially noticeable inside the dome where there's a very patchy pattern in the red light. If you crank up the strength much more the pattern becomes very pronounced. Darktable does do a great job on the color noise removal however. All in all it's not terrible results.
Now, in all fairness Lightroom has been around around twice as long as Darktable and costs a whole lot more. The fact that you can get good results out of a free open source program is pretty incredible. Lightroom was pretty terrible at high ISO NR until version 3 too. Likewise I suspect future versions of Darktable will improve tremendously. But if you're doing a whole lot of shooting at higher ISOs a standalone noise reduction plugin might be a better bet. On OS X it's pretty easy to export the RAW to something like DFine and bring it back into Darktable if you're that picky.