Several points to note:
1. 2005 is different than 2008 in schema and the type of metrics available. 2008 has more information.
2. If your solution does not use a datawarehouse and goes directly to the ezecutionlog, the usage metrics are only as good as the history. The default is 60 days.
A solution exists on codeplex called sqlscrubs to extract all of that information from reporting services 2008 to drive actionable report optimization. It includes the necessary datawarehouse, ssis package and set of starter reports.
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