Swiffy is very similar to a tool that Adobe released earlier this year called Wallaby. The main difference is that Swiffy is web-based, whereas Wallaby is a client that runs on a Mac or PC. Additionally, Wallaby's code is designed to be edited and reused, whereas Swiffy's code is optimized and compressed in such a way that makes editing difficult.
The Swiffy demo page shows off the power of the tool. In addition to converting basic banner ad animations, click-event interactions can also be converted to HTML5. To be clear, this won't convert videos, complex animations or games, but the tool meets its desired goal quite adequately.
Swiffy takes advantage of more advanced SVG features, which means that the only browsers that support the converted code right now are Webkit-based. That means Safari and Chrome on the desktop and Mobile Safari on iOS will support the converted files.