Using NANT quite heavily, but I'm disappointed at the lack of development on this project.  The repository is active, however there has not been a release since December 2007! 

The flipside is that it is really easy to implement your own tasks.  Check out http://randomactsofcoding.blogspot.com/2009/07/writing-custom-nant-task-part-3.html for an example.

One reason I would consider using this is to neatly wrap up calls that I have had problems with, such as SVN and NUnit.

To perform an SVN copy for example, I must perform the following:

            <echo message="Creating Tag Directory in SVN - ${svn.mkdircommand}..." />
            <exec program="svn" commandline="mkdir">
                <arg value="${svn.tagClassLibUrl}/release-${build.version}" />
                <arg value="--parents" />
                <arg value="--username" />
                <arg value="${svn.username}" />
                <arg value="--password" />
                <arg value="${svn.password}" />
                <arg value="-m" />
                <arg value="&quot;new release ${build.version}&quot;" />
            </exec>
            <echo message="Creating Tag..." />
            <exec program="svn" commandline="copy">
                <arg value="${svn.classLibUrl}" />
                <arg value="${svn.tagClassLibUrl}/release-${build.version}" />
                <arg value="--username" />
                <arg value="${svn.username}" />
                <arg value="--password" />
                <arg value="${svn.password}" />
                <arg value="-m" />
                <arg value="&quot;new release ${build.version}&quot;" />
            </exec>


Ewww.