Are sound measuring apps on your mobile phone of any practical use and how do you know?

There are a plethora of apps available for mobile phones that measure, record and analyse sound but are they any good and how would you ever know? it is especially important that you know how accurate a sound level is when you are measuring for the Control of Noise at Work Regulations 2005 or if you are trying to assess an environmental noise complaint, but even if you just want to find out the level of something at home, you may well be surprise at just how wrong a phone can get it!