IE9 may have caught up on some things, but there are a lot of things it doesn't do, that Safari/Chrome and Firefox (as well as Opera in most cases) do do. So even if it might not be quite as hard now to get it to layout properly (it still is not the same as Chrome/Safari/Firefox/Opera, really, things just don't behave the same unless you train yourself to avoid things that IE9 doesn't like), there are lots of other reasons to avoid IE:
■ WebGL (3D)
■ WebSockets
■ HTML5 Forms (validation mechanism, CSS3 selectors)
■ FormData
■ HTML5 multifile-upload
■ HTML5 video (only with certain file formats)
■ CSS3 Transitions (for animations)
■ CSS3 Text Shadow
■ CSS3 Gradients
■ CSS3 Border Image
■ CSS3 Flex box model
■ Application Cache (offline)
■ Web Workers (threads in JavaScript)
■ Drag’n Drop from Desktop
■ SMIL Animations (SVG animations)
■ File API
■ JavaScript Strict Mode
■ ForeignObject (embed external content in SVG)
■ ClassList APIs
■ HTML5 History API
Why are most of these features either already implemented in other browsers or available in development versions, but completely missing from Internet Explorer? Why haven't more of these features made it into the hands of Windows users? Its not because Microsoft can't figure out how to implement or deploy these features. Its because the web platform is fundamentally antagonistic to Microsoft's business model with its dominant Windows monopoly on business software, PC games and operating system deployment.
Those features which enhance the web browser to make it most competitive with the desktop MUST not be deployed to Windows users any faster than absolutely necessary. It is an absolute business imperative for Microsoft to drag its feet and disrupt the web platform as much as it possibly can, because when all web browsers have web workers, 3D WebGL, WebSockets, File API, Gradients, Drag and Drop, HTML5 forms etc., it simply won't make business sense to write applications specific to Microsoft's platform. Pretty soon after that happens, it won't make sense to continue the Windows and Office licensing lock-in.
So the relationship between a web developer (who understands the nature of the web platform) and Microsoft is fundamentally antagonistic.