The official announcement suggest some pretty handy features:
If you go to another page in the same language, you will continue to see translations rather than have to translate one page at a time. And if the page has dynamic content, like Google Reader, you will get translations in real-time. Finally, if you frequently translate pages in the same language, Toolbar will let you translate that language automatically without any extra clicks in the future.
Right now, translation is available for over 40 languages and a Firefox version is coming soon.
The big questions is: Будет ли кто на самом деле это использовать?