Zotac has successfully engineered a unique way to overclock your graphics card. On the other hand most users will be fine by using one of the free VGA overclocking utilities in Windows. The dedicated hardware approach does have its advantages. For example you could change clocks during benchmark runs to optimize the VGA clocks for the benchmark's workloads. Also you can change clocks while gaming without having to quit your game.
Only recently it was discovered that the Zotac software can be forced to run on any NVIDIA card by holding Shift and then clicking the OK button in the "works only on ZOTAC cards" dialog. Still, no go for ATI users. Zotac's Nitro does exactly what it is supposed to, I found it intuitively to use, so even novice users can overclock with it. One of the major advantages is that Zotac gives you full warranty for all overclocking done with the Nitro. I seriously doubt that they can detect if a card was overclocked with the Nitro or Rivatuner for example. So the Nitro could be you $90 overclocking safety net.
At is price of $90 however, this is an expensive gadget that not everybody wants to spend his money on. Still, the Zotac Nitro is certainly a cool toy to have if you can spare the money.