Partitioning is not primarily a performance tuning mechanism. It is primarily a data management mechanism. There are cases where you can get performance improvements, but you need to remember that it's not designed to do that. It's designed to help with large scale data management.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning