My Goodreads ratings are pretty simple. A 5 means I LOVED it, want to own it, would read it again. A 4 for me is a very good book that I quite enjoyed, even if it doesn't grab me quite the way a "5" would. Three means it was okay; nothing that annoying about it but not great. A 2 generally means I thought the book was bad but it had at least one redeeming quality about it. The rating of 1 is reserved for books that I haaaaated. (It's only happened a handful of times.)
When I posted in yesterday's Wednesday Book Club and mentioned I would probably give my current read 4 stars, it got me thinking about how often I've decided how I feel about a book well before I've actually finished it. So many times I'll be less than a quarter of the way through a book and I just know that when I'm finished with it I'll rate it as a "2" on Goodreads, because the writing style is horrible or the plot is ridiculous. I'll mentally rate good books just as quickly. I could tell pretty much within the first 30 pages or so that Alif the Unseen was a very solid book but that it wouldn't necessarily be one of my favorites.
Rarely does a book surprise me so much at the end that I end up revising my estimate upwards. Much more common, although not as much as one might think, do I mark a book down for ridiculous plot twists, stupidity, or just an overall bad ending. It takes quite a bit to ruin a book for me if it's built up enough goodwill in the first half.
Is this a thing that everyone does? Or are other people a lot better at reserving their judgment until they get to the end of a book? I'm curious how others approach it. Does it make a difference if you believe you're rating a book purely for your own benefit versus a public review?