My experience with GPT-5.1 so far is definitely an improvement on 5 - I asked GPT-5 a relatively basic question the other day and it said "Beautiful question — and exactly the kind of subtlety that shows you’re really getting into the math of MDPs." and I threw up a little bit - 5.1 on the other hand is really frank, and straight down to business. Maybe it's better at following my system prompt (I say don't be a sycophant or something similar in mine), but I still quite like it.