I’ve found myself using Tupi more than Pencil these days, simply because I find it more comfortable to use. Though, while Tupi already has an animation testing utility built within, once I export them I’d still like to see the sequence in action once in a while. (Even if I export Synfig projects to PNG sequences)
This is where White Elephant comes in: code.google.com/p/whiteelephant/
Simple tool. Does what it does well. 