Love the concept, but the site itself (which I assume was built using the framework) is visibly laggy (in Chrome, on a MacBook Pro) - I'd be worried about using this for anything where performance was important, and for mobile (esp Android), it would probably be too slow to use. :-(
The next two big pushes are performance and Express (server-side) support. From my own testing OJ compiling, rendering, and DOM manipulation are actually pretty fast. As for the perf issues I am seeing (especially on mobile) I think they are at least partially caused by creating 40+ AceEditors with syntax highlighting and dynamically loading iframes like YouTube videos and TwitterButtons.
That said, lay all the perf blame on me! I can promise it will get better -- though in my defense this is the first release after all. Literally no one had heard of OJ until yesterday=).
