Why not to upgrade to Internet Explorer 9

Not compatible with other operating systems

Internet Explorer 9 works only with Windows 7 and Vista. 55% of Windows users still use XP, 23% run Windows 7 and 11% Vista.
It also does not support Mac OS X or Linux either. Whatever you design on the Web for Explorer 9 will only reach a gradually shrinking minority of the audience.

Other browsers are a better investment as supporting them will offer compatibility on all operating systems. Save Explorer9, all other brlowsers work across operating systems.Explorer 9 is the only browser which manages to be incompatible across operating systems offered by the same vendor.

Lacklustre Performance

IE 9 might win the SunSpider javascript 0.91 benchmark, but is slower compared to Chrome and even Firefox4 on practically every other single benchmark,

In normal daily activity Chrome just feels faster. If you are aiming for compatibility, do not throw away Firefox, which warks on ANY operating system you are likely to find in a desktop environment.

Second class 64 bit implementation

IE9 is faster in Javascript benchmark, but that applies only to the 32 bit version. The 64 implementation is painfully slow and is slower than all its competitors.

64 bit support, one of the main "selling point" in Microsoft propaganda is abysimal, they assume that you are running the 32 bit version on 64 bit windows, which is an admission that the touted 64 bit operating system is simply not there yet in terms of reliable day to day operations by a casual user.. Promises, promises, promises, is all Windows is all about.

Insufficient security

IE9 is still more secure than previous versions of Internet Explorer, but that does not imply that it is nearly as safe as any of its competitors. IE9 does not warn the user of Adobe plugins being unsafe, nor does it check automatically for updates (a la firefox), nor does it implement a browser specific version of Flash plugin (Chrome) which is safer than the traditionally bug ridden and full of zero days exploits versions coming from Adobe. Chrome's plugins actually run in a sandbox which doesn't allow a rogue plugin to attack the operating system resources.

IE9 tracking protection, defaults to allowing tracking whenever there is a security rule conflict. A user can just choose a list author they trust, but have normally no way to audit security whitelists on their own, they need to trust someone else.

Both Firefox and Chrome appear to be much safer choices

Full of HTML Incompatibilities

It doesn't even manage to wrap text properly, if you are visiting blogspot.com sites, unless you use developer tools.

Despite all Microsoft noises about H264 video support, HTML5 support is lousy
In addition, despite all of Microsoft’s noise about supporting H.264 in the HTML 5 video standards wars, IE 9 actually does a lousy job of supporting HTML 5. In HTML 5 compatibility, IE9 is worse than most more modern modern Web browsers: Safari, Chrome,Firefox, which are all better products than IE9.