A day into the iA Writer experiment on my iPad and iMacs, and these are some quick thoughts:
Good: iCloud Support. I like Dropbox, but there are situations where iCloud documents just make more sense to my brain. I haven't had any strange behaviors yet, whereas some Dropbox based apps that I use on both my Mac and iPad do exhibit annoying behaviors. I like that I can have the same document open on multiple devices and that the document itself gets updated in place. I'm not trying to write on multiple devices at the same time, but I don't like getting funny "(saved on …)" files when I forgot to close a window or document on a device. I like OS X Mountain Lion's iCloud document management window within the app, and it's very easy to drag files in and out to file things away in the larger file system (including Dropbox via its OS X Finder Integration).
On iOS, iA Writer also supports Dropbox, but I have not tested it yet.
Good: Simple Interface. This is the first "distraction free" simple writing app that I've enjoyed since the original WriteRoom 1.0 (I didn't like future versions as much). It's very easy just to write, whether in windowed mode or full screen. Writing this in full screen on my iMac right now doesn't feel much different than writing on my iPad with a bluetooth keyboard last night.
The default (and only) font is quite nice on all platforms.
Mediocre: Markdown Links. While inline-style Markdown links are OK, I like the footnote style when I'm just writing things down and want to pile up a list of links to collect later at the bottom of the document. I'm interested in iA Writer as a blogging tool and like that I can compose entries so easily on my Macs and iOS devices, but this little missing feature is annoying. I'm not a heavy Markdown user and don't care about "MultiMarkdown" or other such enhancements, but footer-style links would be nice.
The developers say that such support is coming.
I'm also undecided about my blogging host/platform, so the ability to write in simple Markdown and "Copy as HTML" is a nice feature in iA Writer for posting via other tools that may not have (easy) Markdown support. This need of mine may change.