I have been researching the merits of AngularJS for many months to utilize as a core framework for product I am creating.
There are many aspects of AJS that make it worth while to learn. Yes there is a bit of a learning curve but its well worth it, especially if you wish to have more control on client side capability.
JQuery manipulates the DOM at run time, whereas AJS situates itself within the JS rendering lifecycle. This allows you to teach the DOM new tricks by creating your HTML Elements and Attributes. This is very, very powerful. As what you are able to do is introduce new Element behaviors for whatever your purpose and need. In AJS these custom HTML Attributes/Elements are called Directives. With the ability to craft your own Directives, you are able to build functionality that the current HTML doesn't have, pushing out capabilities that will run on all modern browsers now and into the future. Of the many approaches to inducing new behavior, AJS appears to be the safest direction one could take due to how they have chosen to implement it.
There is a huge performance gain over JQuery in AJS.
I love the simplicity of the two-way data binding, and the separation of concerns in their client side MVC pattern, which as pointed out above provide great testability. There scope object is the glue between the View (HTML), the Model (your Data) and your custom Controllers. The scope provides access to parent attributes and can be isolated at the sibling level, which is important for some reusable templates.
Templates can created and reused across your application which can contain 0 or more custom directives.
I have been using frameworks such as PRISM and MEF but I am finding that AJS has most of the same features that exist in these .NET frameworks but in a 29K footprint. There is rumors that they are working on lazy-loading which if provided will provide for some very interesting LOB type capabilities.
There are a number of UI frameworks that are being built for AJS but you can wrap any 3rd party control lib as needed, given a bit of effort. The trick is to ensure that when these 3rd party controls have changes induced, that you ensure AJS is properly notified using their apply method.
If you combine AJS with MS TypeScript within VS 2012, it provides the ability to manage and build some very impressive projects which will work well for those who are more comfortable with projects within VS.
There are a ton of other reasons to look at AJS, but if you are considering frameworks such as KnockOut I'd highly recommend AJS instead, regardless of it's perceived learning curve. Knockout is a library, AJS is a framework.