I heard about NovelAI's new Kayra model and heard how good it is. And it is flat out good. The new model can produce prose and craft stories very well.  I don't have much experience with it, I have only been playing with it for less than a week, but I can share my experience and what I have gotten out of it.
For reference, I purchased the Opus tier sub to access the latest model Kayra.

What is written below is just my experience with this AI model and a short description of some features that I think are cool. I can't speak to other AI models since I haven't tried any others (except AI Dungeon several years ago).

I also provide some examples of writing that Kayra generated as a proof of concept, but beware I generated those stories on the fly and aren't really proofread. They can be found at the following imgur links here and here. You can see what the AI produced as well my instruct commands and edits.

1. Instruct Mode and Prose Augmenter. Instruct mode is new to the NovelAI update (at least I think it is). How instruct mode works is in the title: You can give specific instructions to the AI and it spits out an appropriate response. For example you can do something basic like: "Describe her appearance. Be sure to emphasize her fat butt and boobs." and it will spit out an often detailed response. You can make your instructions even more complex and it will correspondingly generate more complicated outputs. Instruct mode works great for crafting sex scenes.

Prose Augmenter is another mode that generates more detailed prose apparently. Sometimes I can't tell the difference between the text generated by Prose Augmenter or Instruct Mode, but sometimes I can see an upgrade in writing quality. Fortunately, you can swap between Instruct Mode and Prose Augmenter (or any of the other modes) on the fly.

2. Regenerating and tweaking responses. In conjunction with instruct, it is a good idea to tweak the responses that the AI spits out to gently steer it into a desired direction. For example the AI might write something like "...she bends down at the hip, giving a perfect view of her thong clad asshole." You might accept this response, but for example you may want to have the AI elaborate on this response. So you might edit the above line to read "...giving a perfect of her thong clad asshole. Sweat beads on her fat " and then feed that incomplete line back into the AI.  It will try to complete that line and is helpful for getting out more lines of specific things that interest you.

Tweaking responses that the AI gives you is great for giving you ideas to direct the story since the AI itself comes up with interesting ideas and responses.

3. Presets. Kayra comes with pretty good built-in presets. You should play around with them and read the NovelAI documentation. There are also some really good presets on the NovelAI discord. I use the "Tea Time" preset by Okeri. The examples I provide were written using the Tea Time preset.

4. Lorebook. From what I can tell, the Lorebook is a powerful tool that can be used to help the AI to understand characters, lore, set-pieces, etc. before it starts writing. For example, in the examples provided I have several Lorebook entries containing information about you, your mom, and your bully, including details about basic room descriptions. Having well-developed Lorebook entries provides consistency and depth to your story.

For example, in my Lorebook entries I have information about how the mom is comfortable with her body and likes to wear revealing clothes. The AI remembers this detail when it writes responses.

It is also helpful for the AI to mimic desired writing styles if you write your Lorebook in that style. For example I would write, say, the "mom" character entry using words like MILF, fat ass, mom butt, assmeat, etc. The AI then uses these words as it writes the story, and not just for writing responses concerning the mom, but also details about other women or characters where it would make sense to write in that style.

5. Conclusion. NovelAI's Kayra model is truly fantastic. I have been having a blast with it. I have been using it to expand on my favorite CYOA or erotic text adventure games where I would like some more content. I wholeheartedly recommend trying it out.

There are some kinks with the AI. For example sometimes it can become repetitive if you have it generate a lot of text without enough input on your part. But overall I have been having a lot of fun with it. Stumbling upon NovelAI for me is kind of like discovering a new author, or game, or a fetish with loads of content to look at. It's very good.