We're keeping this here for posterity, but if you're looking for a great SharePoint Gantt Chart for visualizing and aggregating SharePoint tasks, we recommend the IntelliGantt Web Part.
Just a note that build 22.214.171.124 of IntelliGantt and the IntelliGantt Add In for Microsoft Project has been pushed to the web. This one has rich support for Bamboo Solutions Task Master web part. This improves the Gantt Chart experience in SharePoint with parent/child relationships, task dependencies, milestones and scaling.
You might also notice a new sharing option. Because we're still building the support pages it's still on the QT.