I've posted my thoughts on these and re-flashing the ECU before, so wont go on about it again.
The only thing I would point out, is the model of box for the Captiva is £290 (all bar a penny) and it says it will give MPG improvement of between 4-6mpg, which isnt remarkable, if you ask me, as you could get that by removing excess weight, ensure tyres correct pressue, drive with a little less right foot etc...
But the point I'm making, is that I calculate you will have to drive arround 10k miles to see a return based on a 5 MPG improvement. So that would be 10k miles to recoup the £290, then another 10k miles to see a saving of £290.... Not really ground breaking.
Against the reported adverse effects of the way it "fools" the car which is reported (and disputed, but not going into it here... search the forum, think it was OldRoverBoy gave some interesting info in previous posts), to reduce the engines "lifetime" by running the engine above the tolerances specified by manufactures. If that is true, the £290 saving in the second 10k miles, may have been eaten up with extra servicing or repair bills.
Your call... as I said, the forum is littered with posts on ECU flashing, and Tuning Boxes.... I just dont think they are worth the high investment.
If you do buy one, come back and tell me I was wrong...