I ran Gnash on my Ubuntu Handy install for a few months - and it worked ok, and played Youtube videos for a while. But after a while it stopped playing Youtube and all other videos - it would just sit there with a spinner.
So I had to remove Gnash and replace it with the original Adobe Flash plugin.
(Unfortunately although there's a straightforward UI from Firefox asking which Flash plugin you'd like to install (I'd picked Gnash), there seemed to be no UI for removing or changing it.)