In reply to Bjartur i Sumarhus:
they're a piece of cake to use;
for pipes you need some starting point for the signal generator; i.e. you place the signal generator inside (or on top of - less good but works) an access chamber / manhole. The signal reverberates along all the connected pipework and this is picked up on the surface with the scanner.
with a bit of setting-out / line marking paint you should be able to trace out whatever's under ground.
it's reasonably accurate; i.e. more than accurate enough to be able to place services so that you don't plough straight through them with an excavator.
but it won't tell you the difference between an elec cable and a clay pipe.......