Swiffy saved the day for us recently on a project where we needed an animation to support IE7+ as well as iPad. We were able to have our designer create a flash animation and upload to swiffy to test that the conversion worked. We then used the swf as a fallback for when canvas was not available.
The one HUGE drawback for me with swiffy is there was no standalone compiler that we could use and integrate into the site build. So we now have this manual step of converting the files on the (hopefully still functioning) swiffy website every time a change is required to the animation.
I guess it shouldn't be too hard to write a tool which automates that. You could probably even just use curl. I also trust that Google has enough interest in HTML5 adoption that they will make sure that this tool will continue to function.
The one HUGE drawback for me with swiffy is there was no standalone compiler that we could use and integrate into the site build. So we now have this manual step of converting the files on the (hopefully still functioning) swiffy website every time a change is required to the animation.