I use Notion AI. I was already using the free version of Notion for organising and writing all my content. Then they added an AI tool for only $10 per month, so I jumped at the chance. On the whole it is very good, but not perfect. I think it uses OpenAI's API, but possibly with its own modifications, but I'm not sure. There is a huge benefit to having it integrated within my writing and organisation tool. I stopped paying for Grammarly Premium because Notion AI can fix all spelling and grammar with one click.

One thing I particularly like about Notion AI is how it can easily create summaries and lists from existing content. I just tell it to do stuff and it does it, and mostly gets it right.