By many estimates, one in every four users of the 
World Wide Web were browsing with 
Internet Explorer 6 in 2008, over a year since its successor 
Internet Explorer 7 had been released.  In March 2009, 
Microsoft released another major version, yet now two major versions behind, about 20% of all web users cling onto the version that was released eight years ago (in 2001), and was last updated in 2004.
For nearly a half-decade, the software has not changed.  Sometimes that's good; if something works, there is no reason to change software for the sake of change.  However, there are many 
bugs in Internet Explorer 6 that will never be fixed because Microsoft has stopped working on it.
This is bad for the web.  Internet Explorer 7 is not ideal, nor is version 8, but both are significantly better at supporting 
web standards than their predecessors.  The continued use of Internet Explorer 6 is retarding the growth of the web as resources need to be wasted in supporting an out-dated browser that 
never did a good job of supporting the standards of its day, let alone standards that have been developed after its release and retirement.
This is bad for those who use it.  Internet Explorer 6 is buggy.  It provides a sub-optimal experience, and most of the people who use it probably aren't aware of this because they don't have anything to compare it to.  Some users may not have a choice to upgrade to a newer version of 
Internet Explorer because their version of the 
Microsoft Windows Operating System is too old, prior to 
Microsoft Windows XP, which Microsoft has also stopped supporting.  In these cases users are probably not aware of other 
web browsers that are available, such as 
Mozilla FireFox or the 
Opera Web Browser, both of which are technically superior to an of the released versions of Microsoft's offering.