One thing the tester cannot do is tell you how much MDxx (XTC) is in the pill. Therefore the reaction might be purple but there may only be 20mgs in the pill which may not be enough for you to experience a decent roll.
I might also add that most consider 2mg/kg of MDxx will achieve a good roll this will vary from person to person with some needed less mgs/kg to achieve a desred result.
NOTE: The word 'ggod' is very much a subjective term.