6.2. Internet Explorer 5.0, 5.5, 6.0

Available only for Windows (except for some really ancient Mac OS and Solaris/HP-UX versions)
Full of security bugs. (which concern its users, not the web-site developers)
Full of rendering bugs.
Still carries many proprietary extensions that are incompatible with the standards.
Not fully standards compliant. (many details of the standard are missing)
Lags behind other browsers in usability improvements.
"The new Netscape Navigator 4".
The most commonly used browser todate. (especially in Israel)
Can no longer be upgraded except as part of the OS, so its continued userbase will remain a problem, unless it diminishes.

Verdict

If you are running a site which appeals to a large (mainly not tech-savvy) userbase, you still need to support it.
By keeping your site clean of unnecessary problematic embellishments (which is a good idea anyway you look at it), you can best make sure it is compatible with this and other browsers.
You can look at my "Stop Using Internet Explorer" page for an alternative opinion about why I don't guarantee support for MSIE on my non-commercial sites. It has some useful links that explain its problems.