I've had a lot of fun using Gemini's large context. I scrape a reddit discussion with 7k responses, and have gemini synthesize it and categorize it, and by the time it's done and I have a few back and fourths with it I've gotten half of a book written.

That said I have noticed that if I try to give it additional threads to compare and contrast once it hits around the 300-500k tokens it starts to hallucinate more and forget things more.