My experience of Professor Johnson was pretty much exactly the same as everybody else's. I am quite neurotic about my GPA... I got a B+ in this class, my lowest grade at Columbia, and I'm still completely thrilled with the experience. I'm not an English major, so some things which some students may have found easy were challenging for me. That said, I don't think this is an easy class by any standard, so you can rest assured that the amazing reviews for Eleanor Johnson don't derive from unusually lenient grading. I would not describe the grading as harsh either, it seemed pretty fair.
The TA, Emma, was wonderful. Her comments on papers were very clear and helpful, and she created an encouraging, fun, and always on-topic learning experience in her (optional) section.
Professor Johnson herself manages to make every text seem like the most important text in the history of the language. Her lectures are really thrilling and high-energy. She's also very approachable, and responds almost immediately to emails containing insights, thoughts, and questions about the material. She is encouraging, and loves to help students take interesting if unfocused paper ideas and focus them into provocative topics which result in meaningful work.
The exams are straightforward, and she reviews thoroughly before the midterm and final, in class.