I use TextEdit a lot on my Macs, doing almost everything I can in plain text or .rtf documents. I only use Word when I actually need the features. But it was surprisingly hard to find an editor on the iPad that could open and save to a simple .txt. I tried Evernote, but it saves you work as some internal file type that can't really be accessed outside of Evernote itself. Same for my extensive collection of notetaking apps that allow both typing and handwriting.
I wanted an app to work on simple lists, blog posts, and other writing projects in, so that I could also open them in TextEdit or other programs on any computer. Enter PlainText.
It's simple and does its job beautifully. It creates a folder in DropBox, and you can either set it to sync continually or only when you tell it to. (I had it set to sync continually, til my husband was using the computer and got annoyed when it kept telling him that Note.txt had been updated - now I sync manually.) You can create subfolders within the app. The text editor itself is simple, no frills. The document list is in a column on the left (it doesn't autohide in portrait, by the way), or you can go into focus mode and see only the text you're working on.
It's certainly nothing fancy. But it is precisely what I'd been looking for. If I could wish for one upgrade, I would want it to also do simple .rtf files, possibly via MarkDown. I've had a hell of a time finding anything on iOS that does .rtf. But for now, it's serving its purpose very well.