Has any one had any experience with one of these or the likes been offered one on the cheep and need some feedback before i take receipt of it
Cheers guys
Stu F**K IT you only live once

Regardless of how they are constructed (resistor vs microcontroller), the only thing tuning boxes that sit inline with the fuel injectors can do is alter the amount of fuel pressure signal sent. This causes the car to dump more fuel and therefore create more power.

Diesels run very very lean so you can dump a reasonable amount of extra fuel into them to get a power boost, but the downside of these boxes is that your OBD MPG will be waaaaay off because the car won't know how much you've jiggered the figures and therefore will report inaccurately.

Other downsides to tuning boxes is that if you dump too much fuel you'll get lovely black smoke coming out the back. This also means you'll have extremely high exhaust gas temps which can ruin your cats and cause heat issues in your engine.

The worst issue, in terms of costs, is that high EGT's can cause the vanes on the turbos to wear, which means gases will slip past the vanes making the turbos less efficient. This is of course if the high EGT's don't cause the vanes to break and get sucked into your engine.

However a tuning box used lightly can bring a little extra power and sometimes even better economy. However a proper remap can adjust far more things like turning off the EGR valve, adjusting boost maps so you get more out of your turbo and adjusting loads of other parameters allowing you to have a car that can be nice a serene when you're not in a hurry and turbo mental stabbed rat fast when you feel the urge. 
07 Supercharged - Still breaking her in!

Boot soundproofing part 1 - http://www.fullfatrr.com/forum/topic15612.html
Custom exhaust - http://www.fullfatrr.com/forum/topic15296.html
18" wheels for Brembo equipped cars - http://www.fullfatrr.com/forum/topic15052.html

IMHO, a box is not the way to go. Just get a decent remap from a reputable company, and you will never look back.

Money well spent and transforms the rr.

A bit more pricey admittedly but gives far better results.

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

I have my FF Td6 now for one month (about 800 km done with it). From the beginning I felt that the engine was a bit rough (as well on idle and on easy and hard acceleration). Last week I found out that it has a powerbox installed and enquiries at the seller got the information that indeed it is a powerbox uprating the railpressure. This morning I took the box out, and hey what a relief! The engine is much quiter, much smoother and I really can't say it is slow without the powerbox! I'm really glad I took it out. The only thing now is that the gearbox is a bit less smooth on changing gears. Hope the box will learn again how to do it soft Anyone any idea?

And fwiw...I now do understand why the engine had a complete rebuild (including new pistons, new turbo etc) only 10000 km ago at only 200000 km which in my opinion is far too early for this engine: the powerbox had been installed when new... sorry for previous owner happy I didn't buy this great car 15000 km before ; brand new turbo and engine rebuild and no more powerbox : I'm definately good for at least another 200000 km

IMHO: if you want a faster Td6; first do the EGR thing and then a proper remap from a reputable tuner ( I think I won't do a remap at all)

I used a pro bits first for ten months was great tbh no issues and got goodish mpg and went like stink, got a free remap so gave to a mate for his and he loves it no problems runs great and he's had two mots no probs, the pro bitz one is better than most imo, if it's cheap try it for a bit nothing to lose _____________________________________________________
Zambezi silver Vogue Se unique

Only your engine!!!  2006 (56 Reg) 4.4 V8 Vogue In Bonatti Grey (Gorgeous!!)

What you petrol guys know many other ways will do it first  _____________________________________________________
Zambezi silver Vogue Se unique