I've been looking through some diagrams recently created using the sitemap generator. In general, it seems that hierarchies with many broad/flat sections will not be well served by graphviz. Those broad sections become difficult to visualize on the same 2d plane when sibling sections have a lot of child nodes as well. Small to medium hierarchies that are deep rather than broad seem to do better. Using SVGs to zoom through a diagram might work with larger hierarchies.

Here's the example diagrams. Most of these were produced before good SVG output was implemented:

small trees:
"process":http://urlgreyhot.com/graphviz/?q=view&a1=process

vertical trees using colors:
"Capt_Andersons":http://urlgreyhot.com/graphviz/?q=view&a1=Capt_Andersons
"Schooners2003":http://urlgreyhot.com/graphviz/?q=view&a1=Schooners2003

medium-sized radial diagrams
"redheadSAID2":http://urlgreyhot.com/graphviz/?q=view&a1=redheadSAID2
"LexisNexis":http://urlgreyhot.com/graphviz/?q=view&a1=LexisNexis

large trees:
"dasf":http://urlgreyhot.com/graphviz/?q=view&a1=dasf

medium-sized vertical trees
"NSCU.com":http://urlgreyhot.com/graphviz/?q=view&a1=NSCU.com
"Object":http://urlgreyhot.com/graphviz/?q=view&a1=Object

huge trees that become unusable in an automated diagram
"cbot":http://urlgreyhot.com/graphviz/?q=view&a1=cbot