OK, performance is not yet there... But this is something to check out!
I love the debug info and the livestate of the virtual machine.
http://hulkholden.github.com/n64js/
Be sure to have a compatible browser: Click on the 'Info/Videos' link at the top of the page.
Current stable release of Firfox doesn't work, for instance. Prefer the nightly build or use Chrome.