I've been using Scrivener for iOS on my iPad Pro for over a week now. I'm sure I haven't tested out everything it can do, but I'm now confident that the iPad Pro (Using Scrivener) will be my primary tool for writing.
I'll try to summarise what I like (and what I don't) below.

What is Scrivener?

For those who haven't encountered it before, Scrivener is writing software produced by a company called Literature and Latte. This is not a simple word processor, but professional writing software best suited (in my mind) to long-form writing (Novels, academic papers, etc).

Why is Scrivener great?

There are a bunch of articles that explain why Scrivener is great for writers, so I won't go into a huge amount of detail here. Rather, I'll summarise my favourite features:

• It lets you plan and frame out your work logically. Scrivener lets you work in a "cork board" mode, where you can create cards for sections of you project (like chapters or scenes). You can also make this hierarchical - create a card summarising what you want to happen in a chapter, then drop "under" that card and create cards for all the scenes within that chapter. You can add synopses, pictures, statuses, all kinds of things. The best part? Those cards are also text files - jump into editing mode for a card and you can write the actual content. Need to reorganise the sections? No problem, drag and drop the cards into the new order.
• As noted above - it lets you break out your novel into separate files and structure them logically, in multiple levels (folder for Chapter 2, separate files for each scene in the chapter). This lets you focus on the task at hand and break an epic saga into manageable chunks. But don't worry - want to read through the entire chapter as it will appear when complete? A single click of the button and all the files are displayed as one document.
• It can compile your work into a bunch of different formats - including pre-configured professional formats you will need if you want to get published. What do I mean by compile? Click a button, and it can take all your folders and files, stitch them together into a single document, add any necessary front matter (e.g. cover page), and apply consistent formatting. Writing a novel and need an industry standard manuscript? Click compile, select manuscript, boom! (The boom is a lie, it creates a file).
• It can capture all your research, notes, and references together in one place along with your project.
• It can help your workflow - add statuses to your files, such as "needs editing" or "to do." You can then easily display all the files with these tags to zero in on sections you meant to go back to.

There are HEAPS more features, but they are the main ones I use - and I think they make the software absolutely worth the $40 price tag.

What sucks isn't great about Scrivener (on Windows)?

The UI feels like it was created in the 90s. Seriously. I can be pretty vain about software UI (I'm part dragon - I love shinies), and the UI in Scrivener for Windows bugs me.
There is a STEEP learning curve (although the tutorials are pretty good). The software just does so damn much.

Introducing Scrivener for iOS

Relatively recently Literature and Latte released their long promised iOS version of Scrivener. For me, I think it is a game changer.
Literature and Latte have designed the UI from the ground up for iOS, and it is great. Clean, minimalist, useful. I can't say how much I prefer the iOS UI compared to the Windows UI.

The iOS version has most (but not all) of the features of the desktop versions. It has all the ones I care about, although their are some limitations.
What does it do well?

• Great UI.
• Same project files used between desktop and mobile versions, so you can work on either.
• Same ability to structure your projects in folders and files, along with all the goodies (cork board management, reorganising, etc).
• The "quick reference" feature allows you to split screen whilst working (you can have one part of your project open in a side panel whilst editing another in the main editor. This is great for opening up research notes or other chapters to make sure you are getting the details correct whilst working).
• Can compile projects into a bunch of formats and styles.
• Syncs with Dropbox (in particular for syncing between your iOS version and your desktop version).

What are the limitations?

• Cloud syncing (for backing up or moving between desktop and mobile versions) only works with Dropbox - at least for the moment. This is mildly annoying as my primary file service is Google Drive, but not deal breaking.
• Cloud syncing is not automatic - you do need to remember to manually sync the files when you want them pushed to / pulled from Dropbox.
• You can compile to docx, pdf, plain text, etc. It also gives you a number of pre-defined formats, like manuscript. However, you can't customise anywhere as much of the compiled output, and this includes tailored front-matter (cover page, dedication, etc - something you can do on the desktop version).

End verdict?

I just love working on Scrivener for iOS. I'll be using the iPad Pro as my primary writing device from now on, but I can't quite cut the cord from the desktop version yet. For now, I will still need to use my Windows computer to compile more complex versions of my manuscripts.
A final note on licensing: Scrivener for iOS is purchased separately to the desktop versions, so if you want to use it in addition to Scrivener for Windows or Mac, you need to buy both versions. This may be a problem for some people.