Simon Calder: Not all contributors are motivated by desire to help

Many tourists rely upon TripAdvisor – and, as a result, the fortunes of much of the tourism industry depends upon the blend of recommendation and deprecation. But before you plan your next holiday on the basis of what complete strangers have written, consider their motivations.

Many users match the ideal profile: public-spirited individuals who want to share their experiences, whether marvellous or mediocre, with prospective travellers. They are keen to reward businesses that provide excellent value, and to censure squalid hotels and tacky tourist attractions. But there is also a constituency of users who use TripAdvisor in the same way that incensed motorists call the freephone "How's my driving?" number on the back of poorly driven trucks: it is a proxy for making a complaint. Review sites comprise a medium made for the British – at least those of us too timid to demand to see the manager.

More sinister are reviews that are planted to achieve a commercial goal. The greater the power of TripAdvisor, the stronger the temptation covertly to brag about your own enterprise and to slag off your opponents.

The industry is rife with gossip about techniques ranging from friendly encouragement ("If you didn't like the service, tell us; if you did, tell TripAdvisor") to manipulation on an industrial scale using scores of false identities. In its defence, the company stresses that it has systems to identify rogue reviews.

As travellers who rely on TripAdvisor celebrate the 50 million mark, those have yet to try it should embrace the concept enthusiastically. But use it wisely. A recommendation from someone you know and trust is worth 100 strangers' ratings.