I too have been having problems with the HTML5 Trial. Half the time the videos are choppy and require one to wait until the entire video has loaded to get a smooth viewing and the other half of the time the video freezes safari and requires all the tabs to either reload or for the application to be forcibly quit.
I have two extensions running, 1Password and Click to Flash. I added youtube's sites to the allow flash section of Click to Flash but this did not solve the problem. Finally I disabled the extension completely which also did not solve the issue. When I finally left the Html5 trial, the site started working again as expected.
Anyway, you make great products Google, thank you and I look forward to more innovations from you.