I have started to use ACT after 37signals for the ability to added more customizations. However, one feature that I really miss, is the ability to see a running feed of all activity. Act kind of has this feature in the activity and histories of each contact, company, opportunity and even my record, but there is no centralized way to see all of the most recent activity. Currently, the Task List has a filter that you can show cleared activities, but that is not inclusive of all histories. The history tabs are only for the record you are on/viewing, and if you use the "record history" button, no activity is created and therefore, it don't show up in the cleared activity. This provides a great way to see what you and others in your company have done without having to run a report.
I think they should add a another navigation view for "Latest Activity" (contacts, groups, opportunities, calendar, task list, latest activity).