In the process of purchasing my house there were some issues with damp in the report and I wanted to check for myself. This moisture meter performs really well. It came with a battery already, which is a nice touch. You simply touch the ball sensor against the wall, floor, etc and it tells you the percentage of damp detected. It's worth remembering that different materials will read different values and it's worth testing lots of different spots to get a good idea.
For me, it's been very useful, confirming some areas of damp and showing a exactly where the worst spots are. It's easy to use but worth remembering that this meter, like most dampness meters, will be affected by any conductive materials and any metallic objects nearby, such as pipes or electrical wiring.
Is it better than the (much!) cheaper 'two prong' moisture meters? I think so, as those prong meters leave a mark and ideally you want to test lots and lots of areas which you can do very easily with this meter without leaving a mark at all.