I took the time to check out the developer's web site and to download and install - and explore - Darktable. It is actually more like Capture One in it's complexity than Lightroom. In fact, it does many things that Lightroom does not do. The interface is less refined than either Capture One or Lightroom, which is a common issue with open source applications: Darktable is built to run on a number of OS platforms, including several varieties of Linux and OS X - but not Windows. No doubt this is because Linux and OS X are Unix based, while Windows is not. This should boggle some Linux geeks who believe OS X is inhospitable to open source projects. Darktable suggests the opposite: that open source is inhospitable to OS X. If developers are willing to make the effort, OS X is a viable platform. Whether their products are competitive with commercial alternatives, is another question.
Darktable divides its functions into numerous modules, which can be displayed in sets or related features to simplify their use. Modules can be displayed or hidden at the user's discretion. Like Capture One, however, Darktable has a steep learning curve. In my opinion Lightroom is by far the easiest and most intuitive app to use of the three, but Darktable has the distinct advantage of being available for free. This may encourage some serious photographers to try it out. For certain, though, it will not appeal to novice users, despite the price.
Like Lightroom, Darktable catalogues imported images and provides ranking and tagging features and the ability to assemble photos into collections. The database is searchable as well, so asset management is relatively robust.
In limited testing I had no crashes. Processing was moderately fast, given that it is not a native Cocoa app. On the whole, it is a remarkably complete program, clearly the result of a great deal of time and effort. The feature set is extensive and, given the price, in my opinion the app is an excellent value. On the other hand, the interface is bare bones and the program, like Capture One, requires considerable expertise to utilize fully. The user manual is available online and is being actively developed along with Darktable.
In my opinion Darktable is a serious entry in the RAW photo processing and management race. Casual observers may be put off by the interface, which takes some getting used to. Anyone willing to put in the time to learn their way around, however, will no doubt find the wide range of available features appealing. In that regard, I think Darktable is more likely to compete with Capture One than with Adobe Photoshop Lightroom.