Got them through a group buy on P1woc.co.uk.
In day to day use (in the Subaru) I don't often use the record and playback facilities that the Defi setup provides. However, the ability to be able to record and playback 3 minutes of driving and setting the peak boost warning level make them a worthwhile purchase for me.
For example, with the record and playback feature I can watch at my leisure the boost levels the car has been reaching rather than having to keep an eye on the gauge during a 'test run'. The peak boost warning also provides a handy alert if the pressure suddenly goes over my set level.
The other point to make is that once you have a Control Unit then other Defi gauges such as oil pressure, oil temp, etc can be plugged into the back of that and 'daisy chained' so that data and peak levels can also be taken from those gauges.
It's horses for courses, really. As Chris points out a much cheaper mechanical gauge will do the job probably just as well at a fraction of the price, but I personally wouldn't be without the functionality the Defi setup provides.
Hope that helps.