Hey, I am one of the authors of Derby. Knockout and Backbone are pretty different, and they both have good aspects. Knockout's strength is its powerful view-model bindings, and Backbone is targeted at providing an MVC structure. However, neither of these libraries claim to be a complete solution. If you want a more full-featured client framework similar to these, SproutCore is one of the better ones right now.

The reason we started working on Derby, is that none of these frameworks offer a full-stack solution. Backbone and SproutCore will provide better structure and some automation of client-server data syncing. However, they still require manually creating appropriate server routes that handle AJAX requests and communicate with a database. They may provide client-side routing, but these routes are totally different from the server routes, so if you want to use the same URLs, you have to duplicate routing code. SproutCore uses Handlebars, so you might be able to render the same templates on the server for faster page rendering, and non-JavaScript browser or search engine crawler accessibility. However, this will require writing a bunch of custom server code to pull out the right data and render the templates. In addition, none of these frameworks provide tools to deal with data conflicts. Conflicts are inevitable in a multiple users realtime application, especially when supporting offline.

Derby's focus is to unify as much code between server and client and to provide automatic realtime syncing among all models and views. This approach requires less code, delivers faster load times, provides instant client-side rendering from full pages to individual data bindings, and supports offline by default. It's a work in progress, but let us know what you think about it so far.