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.