Vibin Discussions: do you continue an average series or give up?

I have no patience for so-so books. I can DNF a book at the drop of a hat. I figure, why waste my time on a book that's not holding my attention, when I can be reading something better? But then there's those kinds of books, the average books that you like.. but also didn't. It's a 3 star book, maybe 3.5. You're conflicted, because there were good things about it that held your attention enough that you were able to finish it, but you found the MC irritating or the plot lacking. So it's kind of a mute point. And then the question is: do you continue with the series?

That's what I'm struggling with right now. Two trilogies come to mind when I think about this topic: The Kiss of Deception by Mary E. Pearson, and The Winner's Curse by Marie Rutkoski. Both had ridiculous amounts of hype surrounded it upon their release, and I was left wondering if something was wrong with me. Both were so average, bordering on boring. I had issues with the slow plot and MC in The Kiss of Deception, and found The Winner's Curse outright dull. I continued with The Winner's Crime because I had already bought it, and finished it with the same feelings, albeit with a bit more irritation.

So here's where my struggle comes in: spend the money on the next book, and have a 50/50 chance of enjoying it? I usually only consider this because I don't like having unfinished series on my shelf. I don't like not knowing. But then again, I don't like wasting money on books that I'm not going to really enjoy.

So what do you do? Cough up the money just to say you've finished the trilogy/series? Or do you say "Hell no!" and let the story sit unfinished on your shelf? I would love to hear your thoughts!