This is great when the visual editor is on.
I had a peek inside, though, before I activated it and saw these problems:
1. It has a function called "quicktime_post" which will conflict with another plugin with that name, if it was also active.
2. It works only on the FIRST youtube bracket pair on the post. The next ones are unconverted.
3. It relies on the correct sequence of parameters on the YouTube URL:
poster. I saw some youtube URLs that had other parameters, like
search, and had them in another sequence. This breaks the plugin.