At first I was doubtful about testing my JS code, but nowadays I do enjoy it much more that testing the Rails backend. I use my own gem guard-jasmine that runs the specs headless on PhantomJS and it's a real joy! My whole spec suite with over 1000 specs runs in under 3 seconds. I use SinonJS for faking AJAX calls to the backend, but that's just a small subset of all specs since most stuff isn't interacting with the backend.
