The only benefit I can to see to IE9 is full HTML5 support and all graphics rendering is done by the GPU instead of CPU, so it should be less resource hungry.
As we are all only too aware with MS though, "full support", if it's anything like their previous support of w3c xhtml standards, actually means, "fully in support of what we think should be the standard and therefore you should write separate css for Internet Explorer." It really is the retarded child in the physics class. It doesn't have a fucking clue.
There has never been a single MS browser that accurately "interprets" HTML and CSS (OK, maybe IE 4,5 & 6 with deprecated HTML 1 & 2). It does what it wants and fuck everyone else. If it really does support the full w3c standard, I'll eat my eyes. And my self-righteousness.