I spent some time testing Gnash and swfdec and while both worked ok for general use neither was consistently good enough to replace the closed-source Flash. With both some video sites would work while others would fail miserably.

I don’t want to in any way diminish the work that either of these teams has done, they have come a long way and if I didn’t have a need to visit barney.com and other kids sites I would probably use gnash or swfdec without any issues.

Here are the positives to using Gnash or Swfdec:
1) NO MORE TALKING FLASH ADS!
2) You can set it to pause animation at start so you don’t have to see the flashing banners unless you want to.
3) Security: Adobe Flash has recently been the target of a large number of attacks, by not running Adobe Flash I avoid that mess

I will keep checking in on Gnash and Swfdec for the next year or two and contribute bug reports and testing when possible. These projects are too important to be ignored.