For better or worse, I'm often generous when it comes to my ratings. I can't really remember the last time I've wanted to give a book something lower than a 3. (Maybe my superpower is picking out good books? I'll keep it.) Rating is so subjective as to be almost pointless, but here's my system anyway.
5--Highly recommended: Writing is excellent, really enjoyed the story, thought it had something to add to its genre. I was excited about it and wasn't disappointed. Possibly still has flaws, but doesn't disrupt the reading.
4--Recommended: Writing is just ok, but still really enjoyed the story (or the other way around--writing is really great, but the story is just ok). Maybe missing that extra something, or didn't really contribute anything new to the genre. Some flaws that may have disrupted the reading.
3--Recommended for specific groups: Writing is just ok, enjoyed the story, but was just left wanting more. Typical of its genre. Flaws that disrupted the reading.
2--Not recommended: Writing is poor and story line is dull or trite. Nothing new to add to the genre. Flaws abound.
1--Don't bother: Writing is poor and story line is boring. Probably couldn't finish it. Not my cup of tea.