I was asked to referee a paper for it. That was my first exposure to it. I don't remember much, but I think the topic was in my field, so I said yes, but then when I got the paper, the techniques were totally foreign to me. Yet, the reviewing instructions were, "Review it on technical merits only, not on the novelty or importance of contribution."

On one hand, i appreciated that they wanted to be open about accepting anything that was technically sound. On the other hand, there had to be some sense of contribution, right?

In any case, it turns out that I actually lacked the expertise to review it from a technical perspective. But I had already agreed, so I did my best. Can't remember what the outcome was anyway.

So on this incident, it left a poor impression of the journal on me.

There was a paper that explored social causation of gender dysphoria (as opposed to psychological or physiological) from a researcher at Brown University that stirred up some controversy and got published in PLOS ONE because I think it couldn't get published anywhere else. In my effort to understand the science behind transgenderism, I read this paper. I thought it was a pretty decent paper (for the first paper to explore a topic from that angle, anyway) that raised important questions about the causal factors for transgender identification among teenage girls. IMO it shouldn't have stirred the controversy that it did, and I was glad I got to read it.

So that left a positive impression of the journal on me.