But soon the problems occurred, the uploading speed of Copy is much slower compared with Dropbox, and what makes things intolerable is that the Copy app keeps checking every single synced file every time I power on my Mac, despite there is NO change at all. I am not exactly sure whether this is an issue related with the location of the sync folder or not.
I have roughly 80 GB music and other stuffs synced by Copy. After start the Mac, the spinning noise of the external hard disk can last hours or even whole night, which makes me finally decide to stick with Dropbox.
So from my personal experience, I think Copy is not well polished and optimized. Dropbox, if you don't mind the price tag, Dropbox is the best choice at this moment.
Updated 2015-09-12:
The newly announced iCloud 50 GB/ $0.99 plan seems very reasonable. Hope Dropbox may launch similar intermediate plan in the future.