The value over the previous versions of IE is that IE9 implements a huge number of html/css/JS fixes, so it is much more like FF/Chrome/Safari than IE8.
For FF/Chrome/Safari users this means that it becomes much more likely to see web-sites use html5 technologies and leave IE6-8 behind.
So if you create intranet web-sites, then a huge amount of pain is gone when the company upgrades its IE install base to IE9. Personally, I'm looking very much forward to this, and I'm able to tell my customer that this will save valuable time for me (i.e. money for them).
It's probably better than IE8, and if you just use a standard Windows PC then Microsoft Update will perhaps give you IE9 automatically? (Sorry, I don't use Windows enough to know whether browser updates are automatic.) So the common non-computer savvy user will benefit.
Browser updates from Microsoft have ALWAYS been opt-in and that is the reason you don't see the same adoption rates like with other browser manufacturers.
As a web developer, it's my biggest gripe with Microsoft.
If you have Windows set to automatically update then "critical" updates will be install without prompting the user. However, when it attempts to install IE it will always as for the user's permission. I feel that it's a bit different than a regular hotfix.
The least common denominator has been increased. In a few short years, hardware accelerated HTML5 will be expected. 95% on ACID3 will be taken for granted. Really fast JavaScript, ... you get the idea.
Sure some people will hang out with IE6 for another 5 years, but most sites can now be optimized for much more functionality.
Finally, for those people(companies) that will only use a Microsoft browser, they now have a really good HTML5 browser.
The key phrase in your question of course is over everything else on the market. The value of IE9 is that it will get more people off IE6, 7, and 8. Certain types of web app will be much easier to produce once (eventually) they only need to support IE9+.
Someone please enlighten me (this is not sarcasm... I'm asking): what is the value of IE9 over everything else on the market?