These days (in fiction) I mostly read in the indies. It's pretty rare for them to have a star rating on the ebook store I use and I don't use GR because it's owned by Amazon. Since there are so few reviews, when they do have a review or star rating, it's easy for one person with a sour opinion of the work to push the average rating low.
Once upon a time, Amazon's review system was worthwhile. You could save yourself from buying a fire hazard coffee pot by reading the one star reviews. In that period of time, I learned that most reviews are bad. Not that they're necessarily rating the product lower than it deserves. They're just bad at giving other customers a clear idea of the strengths or weaknesses of a product.
That was back when they were all definitely written by humans. The situation hasn't gotten better since a large percentage of the best rated reviews are blatantly slop. As others in this thread have pointed out, star ratings are too easily gamed by people with money to pay for it to be done.
As a reader, I think you'll have better success checking what's available in the preview than you will refusing to read anything below four stars. It's not necessarily even about what's good and what's bad. There's plenty of classics that have great ratings that I personally hate. They're not for me and that's ok.
EDIT: You also asked how I dish out stars. I publish stuff and the TOS for the stores generally takes a dim view of me rating others' work since they assume I could be colluding with other authors or trying to sabotage competitors. I don't use star ratings.