One of the big changes in 1Password 5 for Mac and iOS is a brand new iCloud sync engine. This change is a huge, order-of-magnitude-improvement over what we had in 1Password 4, but it came at a cost. I would like to explain how we arrived at this decision. - AgileBits
My 1Password data currently still lives in Dropbox since I had some bad experiences with iCloud Sync in 1Password 4. The new sync engine sounds very promising so if CloudKit holds up to its promise, I'll probably move my 1Password data off Dropbox and to iCloud. Not because I don't trust Dropbox, but out of convenience:
I've got two-factor authentication enabled on Dropbox, and my 1Password data lives in Dropbox. So when I install a new device, I need to install Dropbox with a very complex password that's stored in 1Password, which is only accessibly once I can get into Dropbox. It's kind of a chicken-and-egg problem, which results in my copying a passowrd from another device.
Compare this to 1Password with iCloud syncing: boot your new device, enable iCloud, install 1Password and boom, it's installed with all your data there.