You also have to be careful with the really cheap chips. What they do is outright dangerous to your engine.
They tap in to the throttle assembly to get the pedal position and the fuel rail pressure sensor to get the current rail pressure. The pedal position is subtracted (simplified maths, it's more complex than this) from the rail pressure and the modified rail pressure signal is sent to the ECU, which sees a lower pressure than expected and ramps up the output of the fuel pump.
It's dangerous because this is ALL they do. They don't look at EGTs, they don't consider air flow, they just pump more fuel in. Anyone with a DPF-equipped vehicle would be seriously compromising their DPF with all the extra soot being created by this.
The cheaper chips (I am NOT referring to DTUK chips, but things like the No-Limits chip that retails for around $150) are really very simple and this is truly a case of you get what you pay for. While I'm slamming the No-Limits chip, I may as well keep going - these things have around $5 to $10 in parts (depending on where you source them) stuffed in a $3 project box. If anyone in Australia is selling these for $1000 then I'd be seriously questioning why you'd buy from them - they're outright thieves!