Without knowing exactly what machine it is that you have, it's impossible to say what gains you can expect with this, or any other tuning system out there. A lot of that comes down to how well the dyno technician does his job, and also, the state of tune for your machine. What you will notice, at least with a competent dyno operator - is that you'll have improved throttle response throughout the entire RPM range. Furthermore, since modern engines are really well-made compared to machines even ten years again, you probably won't gain so much at high RPM. To do that, you'll need to open up the engine and do some really expensive work (deck the heads, flowed intake/exhaust ports, valve job &etc.).
That said, it's the low-end to mid-range power that you really use and, consequently - where you'll probably receive the greatest gains when your Power Commander is tuned by a good dyno guy. Unless all you're doing is drag racing, this is where you'll need power the most, and the PCV will deliver in this particular range.
