I really have high hopes for Synfig. The thing is, I can't get the damned thing to work! I have tried countless times for a while now with self-compiled versions, Ubuntu packages, etc., but although it starts up it doesn't actually let me do anything! I used to think that it was just some really ridiculous user interface issue, and I just couldn't work out how to use it, but I recently found a beginner's tutorial PDF on their site, so I had another go. Start up Synfig, check. Make a new project, check. Click the circle tool, check. Draw the circle... Argh it turns out I was doing everything right the first time, just that the program doesn't work!
I really really want Synfig to work, but at the moment I am forced to use the proprietary Moho to do any computer animation, which is a real shame.