When I'm not sure if I would enjoy a certain doujin, I check the comments to see if people thought what was promised by the tags was well done, same old, same old or somehow original with a twist.
I don't gain anything from people simply stating they didn't like the pairing. I can always tell from the tags what pairing a doujin is about. It's a bit like complaining the doujin is done in manga form and not, say, a video of actual people miming the plot or a recording of guttural song.