Ya I realize that w3c is not yet a standard but that chart clearly demonstrates IE's willingness to move forward on anything and it's always been this way.
I'm glad it works for you but I've yet to see any Webmaster happily embrace it.
I don't have a 20m+ Customer Base to work from but I do have experience from being an ex-Mod at v7n.com and over 8 years trying to have code render the same over the big 3 browsers.
I think its what you're looking to do with it. If you look at our products they use Javascript, Ajax and CSS and a few other things here and there.
When we check things like usage statistics, page views, regressed bugs, new bugs IE9 is actually the one that has the best statistics as a %. IE8 the worst, because it requires allot of specific programming for just the browser. That is really the core difference between then. Additionally IE9 might not have all standards adopted but the standards it does officially support, it supports well; in some cases much more closely to the letter than some of the webkit based browsers.
Im not an IE9 fan boy, I use Chrome for a number of specific reasons (Google accounts, multiple PC's, memory management/sessions never have to be closed etc), but I can see where IE9 stands up and from a practical purpose when producing ecommerce sites, we don't have to treat IE9 any differently than Chrome, Firefox or urg..Safari. IE8 we still have to treat as a specific edge case because 18% of our customers still use it.