I use it regularly but I picked it up during the launch LTD so my opinion on it may be swayed by the fact I don't have to pay monthly (apart from my 50% off subscription to the Pro bolt on). My opinion is that it's an incredible tool that is pretty much a one stop shop for anything SEO content related. The AI writing tool is solid, and does allow for you to create your own custom AI writing prompts or buy prompts from others. There are quite a few AI templates that were developed by Matt Zimmerman (Zimmwriter developer) which are worth using. The built in full blog AI writer will always write content which is well optimised, but it's not as good as other full blog writing tools.
Where it really shines is in the SEO tools. The integration with Search Console is great for finding content gaps, the Wikipedia concept map tool is great for coming up with a content strategy for topical authority and the topic finder/outline builder are decent tools which give some information on search volume and what people are asking. In the article optimisation screens it gives a lot of data based on the search results that can really help you understand the article length that you need if you're going to rank. It also has a lot of information on people also ask and Quora/Reddit questions that can help you find useful points to help answer the kind of questions people are asking. The statistics section can be useful too if you don't know how to back up some of the points you're making, these statistics are scraped from articles, so it really saves time in research. The actual optimisation tool (which tells you words/phrases to write in the article to help make sure you rank) is easy to follow and seems to work really well from my experience. If you use freelance content writers then the Content Briefs section can be really useful, it can come up with the content brief for you and you can just drag any useful information into it before you send it to the writer to write your article.
I usually use it to improve articles I've written/AI written elsewhere, but you can do everything you need to within Frase. It has a chat interface and an AI tools section built into the document writer, as well as a "Write For Me" button to write from within the document. It is currently only using 3.5 for content writing though.
There's a pretty thriving community of Frase users on Facebook and the founder is still heavily invested in the community and the product. There's currently an update of Frase round the corner which is in beta testing (I have access as a tester, although I haven't had the time to test it, really), so it is a product that's very much alive and is a mature product.
If price isn't a sticking point, it's definitely worth it. But if it is a sticking point, then Neuronwriter is still on an LTD on Appsumo and depending on the amount of credits you need, is around 2-5 months of the Unlimited + Pro add on depending on how many credits you need each month. People seem split on whether Neuronwriter or Frase is better, but they both pretty much do the same thing. If I missed the Frase LTD I'd probably have purchased Neuronwriter.