I have one that has proved useful; it identified a crank position sensor fault on my other half’s car and a number of other faults. However on a colleagues BMW with an illuminated engine warning light, the scanner sees nothing wrong.
In use, the normal procedure is to use the reader to cancel the error code – if it returns within a short while it is fair bet that it is a genuine fault.
In general, the more you pay the better the display facilities/options.
If you want I will place a link to the model I have – I reckon it well worth the £30 or so.