A very good browser, but not a great one. I have a few gripes with it:
1) UI: IE9 looks brilliant, unless you enable extra toolbars like the favorites bar. These ones don't have the transparancy of the rest of the UI and thus stick out like a sore thumb. In FF and especially Chrome, these toolbars look like they are a part of the browser, not just something that's sticked on top.
2) Ad ons. Or rather, the lack thereof. (although in corporate environments, that might be a good thing). I'd look to have an ad blocker and a bookmarks sync features that don't require seperate programs to be installed.
Give IE9 these features, and it's the best browser on the market. Now it's just something worth considering.