Haven't used it in anger yet  (still beta, getting nowhere near production environments ). But from testing in the lab its a much cleaner interface, no empirical evidence but feels more responsive.

No additional functionality at the moment, but from listening to the weekly Tenable podcasts it seems that replacing existing functionality is only the first step, other improvements are in the pipeline. Although I'm not expecting too much in way of new features, if they put too much useful stuff in the Nessus Scanner, it would detract from the (comparatively much more expensive) Security Centre.

Always like new and shiny though toys though, and the move away from Flash should help those with portable devices of the flash-less persuasion...

I like that they added it to the currently installed version and you just need to point the browser.  I have a few scans that I need to run so may use that interface to run them.  It is pretty slick looking and fairly responsive.