Phrased more broadly - everyone has biases in their opinions. Make sure you know what their bias is and take it into account when listening to them.

Widgets-R-Us has a strong financial incentive to make their Widgets study as advantageous to them as possible. Widgets-R-Evil has a strong incentive to blast Widgets-R-Us's study as invalid and make their own study as negative as possible, since their reason for existence (assumedly) is convincing people that Widgets are evil.

The truth is A) usually without a direct incentive of its own and B) usually somewhere in the middle.