The idea behind them is that it's a minimal obstruction, but at less than full throttle it will make the bike behave as if it has a smaller carb. At WOT the "blade" is insignificant, so you get the benefits of a smaller and a larger carb without the disadvantages of each. I've never used one but the idea seems sound.
I've used them on my dirt bikes.
It helped with flame-out issues - where the bike suddenly dies when you chop the throttle - in tough sections out in the woods. It was a noticeable improvement in that regard.
Other than that, maybe a 'little bit' cleaner in initial pick up of throttle, but I wouldn't say there is a noticeable increase in power.