No, I de-prioritize reading 5-star reviews. I rarely more than glance at them. I prioritize reading 4-star reviews, figuring they're much less likely to be paid/fake.

I've also heard speculation that for cheap products, competitors will buy it & trash it in the review, not sure if it's true or not though.

I think GP had it stated correctly: if you want honest opinions, look at four star reviews. Five/one star extremes tend to read overly emphatic and aren't as likely to include both positives and negatives, so as a consumer (and perhaps as a platform hoping to show "good" reviews), these more moderate reviews will likely give a more accurate picture of the product, and thus could get priority for display. That said... as soon as you do that, obviously you're going to see four stars getting gamed as well.