I used to run a Seat Leon 1.9 and my wife a Mk5 Golf 2.0TDi, with a Rapid Chip. There was a significant increase in performance and fuel economy on both.
The chip itself was a retro fit black box, which came with an adapter wiring loom. It left all the original equipment in place, so if you needed to remove it for servicing or any other reason, it was a less than five minute job.
Both cars were immediately quicker, the claimed 20% increase in power being pretty much bang on. I did some back to back testing and saw a decrease of a second in 0-60 time and improved flexibility across the entire rev range.
If I wasn't being heavy footed, then both cars would consistently return 10mpg better on an average journey (based on my wifes journey to work, 20 minutes on an A road, some stop start and running).
The down side was the box wasn't cheap. They cost be about £400 each but in my opinion, they were worth it.