PSA: If you're not already on it, have a look into pixate. It's pretty awesome.
I'm still using [http://www.getcomposite.com/](Composite) to preview UI as I design, but for animation and UX/UI concepts Pixate has been really helpful.
In the first afternoon of using it, I set up a base template, and prototyped three different pull to refresh concepts - and felt super productive. All from a web app which was impressive, and it deploys to a native app on device, which is incredibly important.
I love form, but it's time intensive, and can feel a bit like stumbling in the dark.. so It was awesome to be able to set up a few concepts complete with animation without too much effort, and no need for dev work. As a bonus, handing on the timings and a prototype to the devs made building the thing 100x easier.
The GUI and drag and drop of it is good, but admittedly the UX can be a little unintuitive. The ability to copy/paste/duplicate/name animations and states is really needed, as it gets a little tiresome having to input the same thing ad nauseam for multiple objects, but overall it's not too bad. Also missing the ability to conditionally return an object to its initial state, which is a big one. Pixate if you're listening I would love to give some feedback, as with some improvements its a product I would be more than happy to pay for.
All in all its a free product thats really helping me prototype and I thought I'd share in the hopes it helps other.
You can do some pretty cool shit: http://www.slatestudio.com/blog/p/complex-interactions-with-pixate