Krankin hit the nail on the head. But you've raised the subject and I hate to see people wasting their money on snake oil so I'm jumping in.
The idea of swirling the intake air is a good one, because it would allow for a more evenly distributed fuel/air mix in the combustion chamber just before the inlet valve closed. Once that happens, the piston starts rising again and what you've hopefully got is a nice spread of compressed fuel/air across the top of the piston ready to be ignited. So, for carburettored normally aspirated engines, the hiclone MIGHT do some good. You'd do better by changing the angle of the inlet ports to direct the inlet air in so that it swirled on its own - and if you've ever pulled an engine apart, you'd see that as far as is mostly practical, that's how they are anyway.
Take away the fuel from the inlet stream (ie use direct injection) and you're not achieving anything at all with it, because there's nothing to mix until the fuel enters the chamber which should be close enough to the top of the stroke that the air's mostly all compressed anyway. If you're having issues with the fuel/air mix being unevenly distributed in a direct injection vehicle then your injector is not right, get them cleaned.
Add a turbo to the mix and the thing becomes an inlet restriction. The air entering the turbo is compressed and spun out - the turbo charger uses what is called a "radial compressor" which sucks air in and by a similar principle to the rock on a string swung around your head, the air leaves the compressor vanes at right-angles to the inlet stream, is collected and forced into the inlet manifold. Swirling the air before it reaches the turbo is absolutely pointless because of the way the turbo alters the air flow.
Swirling the air AFTER the turbo would have to happen AFTER the intercooler, too, as the fine tubes in the intercooler would quickly stop any swirl effect there as well. Since the air is under pressure at that point, there's not a whole lot to be gained by swirling it some more, since it's going to be violently pushed into the combustion chamber rather than sucked in by the downward motion of the piston. Then, don't forget, this is just air, not a fuel/air mix, because most of our cars use injectors not carbys.
Would I buy one myself? No. There are better areas that I can spend my money. I'm thinking furry dice might improve my fuel economy too, so I might try that. The theory there is that I want to go away for a weekend, so I start the truck, sit down, become hypnotized by the swaying of the dice and don't wake up until Sunday, then pack everything away again ... I've saved a whole heap on fuel, wear & tear, greenhouse gas emissions ... yep, them dice are going to be on my shopping list.