I personally use Rawtherapee which seems more advanced(but a little more difficult to use for beginners) than Darktable. Ιs there any reason to use Darktable apart from ease of use?
Darktable's workflow better met my needs at the time I was picking between them. In particular, Darktable was better for processing a few hundred images in a couple of hours right after shooting. Darktable's integrated image management was the key difference. Darktable's ability to offload processing onto GPU's was also a factor. GPU's make Darktable much much faster than without.
I've got a photoshoot booked for tomorrow, so I'll try this out and see how it compares. (An average shoot for me results in 200-400 CR2 images to examine/reject/process.)
Darktable supports masks and parametric masks for almost all operations. RawTherapee on the other hand has a wider range of tools and support for profiles and raw formats.
RawPedia's "Features" page (http://rawpedia.rawtherapee.com/Features) doesn't mention photo management, and I didn't find anything by searching for "catalog" (Lightroom's name for a collection of managed photos), etc. It'd be great if the site had more info about that aspect of the app.
It doesn't have "collections" or "catalogs" per se, if that's what you're specifically looking for.