I recently used to tool to drive in a few post spikes. It works by inserting it in to spike and acts as a platform for the mallet/sledge hammer to hammer on to.
It makes driving spikes in much easier and I would definitely recommend getting one if you need to put some fences up.
The only slight negative was the metal rod that slides through the tool can start to bend as you hammer the tool. Every now and then you may need to remove the tool, and tap out the bend in the metal rod.