i use both. CC if my library PC is awake, C cloud if it's not.
given that C cloud is not accessing your actual live calibre library ( unless you are daft enough to keep that in dropbox), and not in direct contact with your calibre program, it's hard to see what "features" a competing program could offer that would be better
( just my 2c)
I think my only gripe would be that it lacks the ability to sort on tags , or on series. I forget which one it can't do , which shows how often I use it
an ad-free version in the amazon store would be nice, also, but I guess C cloud sales to date do not justify the effort...