Manton Reece: iCloud vs. the web:

"If you're a developer considering iCloud support, just make sure your data fits there. Ask yourself if your data is all about your app, or if it's bigger than your app. Developers who are willing to take a risk on building an open API instead of iCloud could see new opportunities: web-based views of their data, compatibility with other apps, and syncing on the Mac outside of the App Store."

Federico Viticci also notes:

"I know of several developers who tried to implement a single iCloud storage for some of their apps, and got rejected, as Apple doesn't like the idea of different apps from the same developer sharing iCloud documents."

Spoiler alert: VoodooPad 5 doesn't use iCloud syncing for one simple reason- it's currently impossible to share documents between two users over iCloud. Combine that with the horrible UX on the Mac and making it super annoying to beta test- well, I'm using Dropbox instead.

Speaking of Dropbox, their new /delta api is wonderful. If you've built syncing on top of Dropbox previously, it's worth your time look into the delta call.

Update: Michael Tsai also adds:

"When Steve Jobs said that Dropbox was but a feature, perhaps he was revealing a blind spot. The cloud, or the Web, is more like a platform. If Apple doesn’t want to provide the APIs, developers will find others to build on."