iCloud's online storage in Yosemite seems to work more like Dropbox - hopefully when it goes Apple will also announce an increase in free capacity
I don't really understand iCloud Drive. I use Dropbox as a sort of virtual memory stick for keeping documents and files I DON'T want to exist physically on my laptop because I don't need regular access to them. iCloud Drive seems to save your documents and then physically store them on your Mac as well as in the Drive (albeit, keeping them synced), this is different to Dropbox which gives you the option of having the Dropbox folder installed on your computer or not.
I'd be happy to switch to iCloud Drive but if it's going to force me to store the files I put in it on my computer, it's of no interest to me.