Diesel tuning chips, do they work

[PJbiker]

I've read about these chips and found several sites selling them from between £130 and £500, promising 30% more power and ~8% better fuel efficiency. Have considered getting one for my 100PS 2004 focus 1.8tdci but come from the "if it sounds too good to be true it probably is" camp.

Does anyone have any experience of these, good or bad?

eg.
http://www.carperformancepartsuk.com/shop/prodshow...