there is a fantastic report called 'Converted Leads' - it measures the number of leads that have been converted to a company and contact from the 'Lead Created Time'.
So I can look back at all leads that came in a particular month e.g. last September and see what percentage are converted to Companies and Contacts over time.
Unfortunately it is a system report and the Help Desk says it can't be duplicated or edited. It would be great to add at least a Sales Amount column and add it up. Or even work out the average number of days it takes to convert a lead to a Company/Contact