I am a longtime OmniOutliner (OO) user on both macOS and iOS, but I'm keenly aware that it is increasingly difficult to justify relatively expensive software to many users. The Omnigroup is one of a handful of developers who have taken both macOS and iOS platforms seriously, but new features typically lag in their iOS version. iOS has become my primary platform for the last two years and OO3 brings the iOS version up to date and with key features that I think make it more compelling for serious users.
OO3 is now a free app download with two optional in-app purchases. The basic free version will allow anyone to view outlines, but not edit them; this is great for sharing outlines with those who are convinced they have no need for outlines. The "Essentials" upgrade supports editing with all of the features many would want to create outlines. The "Pro" upgrade adds the full feature set, which allows the user to create and navigate large text databases, along with better control for formatting and sharing, along with other features.
I think the real challenge with this app is to understand what to use it for. It's not a replacement for word processors, nor for spreadsheets, although it can do many similar things. If there were more substantive examples of advanced OO3 use, I think it is likely more people would understand its value. One unfortunate trend with the Omnigroup has been to skimp on documentation, which now exists only in web form, making it difficult to search meaningfully, and which only presents features with few examples. If you aren't interested in figuring out how to use the advanced features, I doubt the price will make it worth it.
So, I would give this five stars for those who can see the possibilities and are willing to dig in and learn it, but reluctantly only three stars for others. Regardless of which version you get, this is some of the most thoughtfully designed and expertly implemented iOS software I have worked with.