It doesn't look as simple as they make out to fit and get the sensors working. I have one of these:
http://www.ultra-gauge.com/ultragauge/index.htm It does the job well and sucks to the windscreen just like a sat nav. There are 74 possible guages and it is also a code reader for £40!
The massively annoying thing is that it does not do boost pressure and if you leave it plugged in it will drain your battery overnight. I learned the hard way...