Berkshelf is worth investigating simply because there's a lot of momentum behind it, even though it came out after Librarian. By "momentum", I mean that since the session introducing it en mass at the Opscode Community Summit in 2012, it's being adopted by the community.
No hard numbers, but the main thing is that Berkshelf could not have really happen without enough standardized community cookbooks. By putting the cookbooks into a global directory, it works more like Ruby gems and Bundler: instead of focusing on your customized (and probably divergent) cookbooks loaded into your project directory, you focus only on the few cookbooks that are specific for your needs. It comes out of a bigger theme in the 2012 Summit, that of creating "library cookbooks" that gets assembled by your "application cookbook". If it's a library cookbook, you don't necessarily need it in your project library any more than you need to import a Ruby gem into your Rails directory during development.
Test-kitchen is also worth looking into, though as far as I know, that still uses Librarian under the covers. Berkshelf support for test-kitchen is pending (http://tickets.opscode.com/browse/KITCHEN-9).
In summary: Berkshelf is part of the overall trend in modularizing these cookbooks and developing tighter engineering discipline.
No hard numbers, but the main thing is that Berkshelf could not have really happen without enough standardized community cookbooks. By putting the cookbooks into a global directory, it works more like Ruby gems and Bundler: instead of focusing on your customized (and probably divergent) cookbooks loaded into your project directory, you focus only on the few cookbooks that are specific for your needs. It comes out of a bigger theme in the 2012 Summit, that of creating "library cookbooks" that gets assembled by your "application cookbook". If it's a library cookbook, you don't necessarily need it in your project library any more than you need to import a Ruby gem into your Rails directory during development.
Test-kitchen is also worth looking into, though as far as I know, that still uses Librarian under the covers. Berkshelf support for test-kitchen is pending (http://tickets.opscode.com/browse/KITCHEN-9).
In summary: Berkshelf is part of the overall trend in modularizing these cookbooks and developing tighter engineering discipline.