I must admit that I liked the first half or so of this episode: the situation between the Brekkans and Ornarans was intriguing and the concept is an undoubtedly an interesting one: what if one culture deliberately addicted another to a drug that only they could provide? Yup, the Brekkans were a fairly horrendous people who displayed villainy on a scale that even Khan Noonian Singh would quiver at.
Where this episode let itself down was the conclusion. It went right downhill the moment Tasha began her infamous 'drugs are bad' speech to Wesley...which was simply cringe-worthy, reducing TNG to some Saturday morning kid's cartoon which always has to shoehorn in some well-meaning but inane moral of the week. It's a wonder they didn't tack on a scene of Picard sitting in his ready room at the end of the episode, addressing the audience: 'well, in this week's episode we explored the dangers of taking drugs. As we all know, drugs are bad...'
The conclusion itself is a huge let-down, with Picard once again droning on about the Prime Directive like some kind of robot stuck in a loop ('the Prime Directive is a philosophy and a very correct one,' he tells poor Beverly who is stuck in a turbolift with him).
'Symbiosis' could have been much better had the ending not been so horribly heavy-handed.