I found Angular was easy to pickup - initially. Once you strayed from the path it became more difficult to understand. It is a cool framework, but I feel like you have to take it on whole-hog or nothing. Also - and this is completely and illogically subjective - it doesn't feel like JavaScript to me. It feels like Flex or I don't know what...
I built two apps with Knockout.js (similar to Angular I think) and many more with Backbone and I can say that it's true that it's harder to do non-standard things in Knockout than in Backbone (I'm talking about view layer). But there is a reason for this: standard things in Knockout are so much simpler and faster to do than in Backbone that it almost always pays off.
That being said, I see no problem at all with using them both at the same time. There is a Knockback project, which I found after I wrote a hundred lines of code helper that does exactly the same thing, and I can tell that the work great together. Backbone is good at the Model layer, it's Model ad Collection classes are very good; Knockout on the other hand is exceptionally good when it comes to Views and Controllers, almost entirely eliminating the latter.
I think that people fear the complexity of using both frameworks in one app, but in my experience it's not that bad. Everyone should give it a try.
