I'm surprised no one has mentioned Tumult's HTML5 animation app Hype . I've found it to be fast and relatively easy when I'm trying to communicate particular animations. I wouldn't want to build a whole prototype with it, but it's great for those medium fidelity animation mockup, has an iOS app that can display what you're working on directly to your device, and is only $30.

Anyone using this? Anyone not using it for a particular reason?

AE is great because you don't spend your creative thinking cycles on implementation. QC, Framer, etc - are great for testing interactions, but really rigid for fleshing out the broad strokes.

I generally use after effects to visualize transitions. I love it, it awesome and it's limitless. You can animate anything, even a shape in 3D. However, web has limitations; you can't animate a shape just easy as after effects. You need to create svg shapes and animate it using a library (i.e. Snap) which can be sometime frustrating and the result doesn't give desired performance. I believe we need another tool to animate any Dom elements including canvas and svg.

I've been using After Effects in my workflow, and it's been really helpful for showing transitions and motion with my developers. I wish I'd taken the time to learn it a while ago.