People stick with Dropbox for a simple reason: they do the syncing thing faster and more painlessly than anyone else out there. If Apple ever gets their act together and makes iCloud actually fast and reliable a syncing service, Dropbox might be in trouble.
What's the actual issue with iCloud? It takes three seconds to turn it on, and select what I want to sync. Done. Backing up, syncing calendars, contacts, and notes to all my devices with zero problem.
Every cloud service needs a "delete, damn it, I really mean it" option, otherwise it seems their sync mechanisms are too conservative and resurrect things forever.