Recently I discovered AngularJS (http://angularjs.org) by some of the guys at Google. I spend most of my time on the backend and have never been a fan of JavaScript, but the AngularJS declarative design really caught my eye. It's so clean and so simple. I think it's brilliant.
A colleague and I are using it, and it's been great thus far. The learning curve is a bit steeper than other similar frameworks, but once you get the hang of it, development is very quick. The Angular guys are very helpful as well.
I'm using it on a project right now. It's my first project with it, but so far so good. The tutorial is quick and straightforward, and it really gives you a flavor for it (http://docs.angularjs.org/tutorial/).
AngularJS is a lot like Knockout, without the ko.observables everywhere.
I've been using Angular in a few prototypes and ran across funky problems using their routing in HTML5 mode with IE9.
Theoretically their router should prepend the hash fragment to the URL, but it gets stuck in a redirect loop and hangs the browser. The workarounds I've found so far are to use hash style URLs or not use a default route, neither of which are optimal. (Edit: updating to 1.0.1 fixes the problem as long as your "otherwise" route is a defined route)
Version 1.0 (they are currently on 1.0.1) of the framework broke their website for IE users, which makes me a little nervous about the level of cross-browser testing that the framework uses. Their documentation is sparse in some areas, which is an annoyance more than anything (the source is fine to read, but be forewarned that it's not a small project by any stretch of the imagination).
On the plus side, not having to litter ko.observable everywhere was nice, their concept of services (for working with REST endpoints) is great, and it's been pretty easy to break everything up into maintainable modules.
Neat. I don't love the ko.observables, but as an alternative to having to have objects fit into a predefined structure/class hierarchy, they seemed like a good tradeoff.
See http://www.youtube.com/watch?v=elvcgVSynRg