We are just planning on upgrading a fairly largish site and SI would have been a great tool to assist in this. The problem is that SI requires that no previous version of ACT! or ACT! SQL Instance be installed on the machines. This therefore means we have to manually uninstall ACT! and SQL from over 60 machines. Since this has to be done manually, we might as well install ACT! manually whilst at each machine.
If the SI could be conifgured to detect previous installs, then uninstall thar version before installing the new version it would be a much more useful tool within the larger install sites we feel.