This looks to be at the better quality end of the spectrum for such devices and may even do what it says on the tin. Plenty of such things are either a scam or just trick the engine management unit into thinking the engine isn't yet up to temperature, thus causing it to permanently inject more fuel. Either way, I'd avoid it. It's much better to remap the ECU itself.
With the remap you know it already works and is optimised across the whole rpm. Add on box, well either a resistor for air mass or other sensor, so what happens when it is cold? Ecu very confused and possibility of limp mode.
Remap is a night and day difference. TDI75 remapped on a 200 mile run, Yesterday got 67mpg driving like Miss Daisy(around 60-65mph). With the small fuel tank I get between 375 and 420 miles till the light comes on. Therefore (at 118k) a decent remap wont hit your economy too hard whereas as box will more than likely just confuse the ecu. Plus I paid less for a map, the software and a lead than the figure quoted on eBay..... Im happy with the performance increase without it being detrimental to the rest of the drivetrain. After all, what use is a load of power with it breaking the weakest link? Plus once youve bought the software you can* swap between standard and modified maps if you sell on..
These types of add on boxes simple modify certain sensor inputs to the ecu
Result is excess fuel injected more smoke less economy and ultimately engine damage
They can never be right across the Rev range or cold to hot engine
Best avoided if you value your engine
A remap is the way to go ( as others above have stated)