I really like Dan Moren's acticle on iCloud from MacWorld.
Looking at iCloud in particular, it seems that Apple's approach to sharing is almost antithetical to the concept itself. The online service rigidly defines what you can share and how you can share it; its primary directive often seems to be to keep you inside Apple’s ecosystem.
Here is how I feel about it:
iCloud is for backups and for syncing data between devices for some apps- a good example would be a podcasting app like Downcast or Instacast.
Dropbox is for files that you want to access from multiple applications/devices/platforms/operating systems.
A good example of this would be Byword, where I write most of my posts, notes and any other type of writing. I could use iCloud for this, but then I wouldn't we able to access it from a device that isn't mine and/or doesn't have Byword installed.
Another example would be my photography workflow. I save incoming photos in Lightroom on one of my hard drives. My final edits however, are saved to a folder on Dropbox so that I can easily access and post them from whatever device I happen to be using.
I also love that I can share anything I'm working on with anybody else by just sending them the dropbox link.
If you haven't started using Dropbox yet, here is a link to signup for free. If you sign up, you and I will both get some extra storage for free.