For now, Dropbox is absolutely First Rate. I'm thoroughly impressed with the service. Its syncing function is also included in a wide range of iOS apps and their OS X versions, such as WriteRoom.
My biggest beef with iCloud is, as has been mentioned, that there is no iDisk-like storage capability. Beyond that, Documents in the Cloud syncing with Lion is a bit gimped at the moment. I'm referring to iWork (Pages) here.
Document syncing is seamless between iOS devices, but I have to use the iCloud website to download and upload Pages documents on OS X. By contrast, Dropbox simply has a folder to allow easy dragging and dropping. Now, there *is* a sync folder for Documents in the Cloud buried in the OS X filesystem, but using it Dropbox-style sometimes results in problems and document corruption. So it isn't a good idea to mess with that.
At some point I believe Apple *will* implement Documents in the Cloud syncing with OS X that will be better than whatever drag-and-drop paradigm is currently the ideal.
At the moment, however, this situation seems to suggest that Apple is encouraging iOS use above OS X use. At least tacitly.