I mostly modify JPG images taken from my Nikon camera. How does Darktable compare to Adobe Lightroom for this purpose?
Darktable handles JPEGs almost as RAWs. It just activates different processing modules by default, and e.g. the demosaicing module is of no effect for obvious reasons. See the Darktables module dependencies diagram. This diagramm is loosely processed from the bottom to the top by darktable. So, the arrows are followed in reverse direction. The user can switch on an off every module, and set parameters.
For RAWs, Darktable activates by default "rawspeed" (i.e. RAW import), "temperature", "demosaic", "basecurve", and "sharpen".
For JPEGs, no module is pre-activated. (No basecurve is applied because this has already been done by the JPEG engine in the camera.)
By and large, that's the whole difference. Darktable tries to do the right thing™ in both cases. You may easily overlook what type of picture you are about to edit.
The "lens" correction module should be used with care because TCA and vignetting correction need proper linear pristine colour spaces to work, but this is usually not the case for JPEGs (normally sRGB). If the JPEG is not yet distortion-corrected, you may well do so, though.
Everything after (top of) the "colorin" module really is the same for JPEG and RAW and can be used freely.
The developers of Darktable seem to shoot in RAW exclusively. This may make JPEG a second-class citizen in Darktable. However, I shoot 80% in JPEG. I use Darktable for 9 months now and it works great for me.
The only thing which is worse for JPEGs is the slower import speed. It is totally sufficient for me, but your milage may vary. Be that as it may, you can test very easily whether the JPEG import speed of Darktable is an issue for you.
Lightroom is not much different. It hides the internal details of the processing of JPEGs and RAWs as well. It doesn't show the implicit processing steps for RAWs as Darktable does, but being more explicit is a general property of Darktable. One may like this or not.
Lightroom mostly refuses to apply lens correction profiles to JPEGs. This is because it may be a bad idea, for example, to correct lens distortion on a JPEG that has already been corrected in the camera. In contrast, Darktable allows lens correction for JPEGs as for RAWs.
This is a non technical answer. I just want to describe my experiences using both.
I use Darktable (version 1.2) on Ubuntu 12.10 and on OSX and the RAW handling is pretty slow, but it handles JPEG processing swiftly and with no processing time. It can get flustered and will often bomb out if you do massive imports of JPEGs. I shoot JPEG for events like balls etc. I have had to ditch using Darktable for this because the crashes made it far too unstable to be used in a professional context. Experiences vary greatly with it though, and the devs are very good at responding to the bug issues in version releases.
On a side note: There is something great about this software though. Having so much power over my photos without shedding a penny is amazing! I would recommend trying your hand in Darktable before purchasing an alternative (if you don't already own one).
I use Lightroom (LR) under windows 7. Although it's processing can take longer (milliseconds on a PC designed for graphic rendering) and you have to commit to an archive collection format (which is generally a pain for migration), I don't see what else you would gain by processing JPEGs with Darktable.
There is far more documentation and tutorials for LR and there is nothing Darktable can achieve that LR can't. You end up being able to control every aspect of it very quickly and your photos will hopefully benefit as a result of this.
More presets are available for LR, making it more accessible to people who just want to swiftly get through a holiday album, or put instagram style filters on a night out! (Probably the most common use of JPEG editing!)
Specifically for JPEGs: The most valuable tools for a JPEG post processing session are the white balance tools. In my experience the LR tools are far superior to that of Darktable, meaning a faster throughput of photos. This is not a great problem for RAW shooting, and therefor low on Darktables priorities of stuff to improve.
The differences in actual editing are negligible. It's not even worth migrating between the two.
LR comes with a price-tag, Darktable comes with bugs. They both do a cracking job of editing JPEGs!
In one important dimension they are identical. Both are non-destructive editors and therefore do not add a compression artifact with each change.