Yeah, already figured it out by poking it until it worked a while back. Thanks anyway thoughTwo ways of doing it:
Code:{xen:if '{$user.user_id} == 42', '<span class="userrank">Rank-for-member-42</span>'}
Code:<xen:if is="$user.user_id == 42"><span class="userrank">Rank-for-member-42</span></xen:if>