Chronosync and Dropbox

It depends on what you want to accomplish, and how you want to accomplish it. Dropbox Backup, in general, is good for MIRRORInG data on computers/hard drives, etc. But it will not let you have a different set of data in the cloud, and locally, nor will it let you control how that data is synced/managed between the two. This is where ChronoSync shines, more on that in a minute. So whatever you delete/modify in Dropbox Backup, for computer, will change on the computer version of those files/folders, and vise versa. Backups of external hard drives, which were just introduced, will not let you edit anything from the dropbox side at all, they are strictly a clone of the local hard drive. Also, recently, dropbox seems to be revoking share access to anything in a backup, whether it's a computer or hard drive (someone correct me if I'm mistaken here), which I really don't appreciate. If I choose to share something in my dropbox, wherever it may be located, I should be able to do that without restriction, but that's just my opinion.

The above explained backup solution that is part of dropbox itself doesn't work very well for me personally, because I don't have the flexibility to do what I want with the source/destination files separately, and more importantly, at least in the case of the computer backups, dropbox makes symlinks, in the form of aliases on Mac, to your documents, downloads, etc on your actual machine, and moves all of your content into your dropbox folder. This means that the OS still sees your documents path at:
/Users/Victor/Documents
But the true path of the folder is now:
/Users/Victor/Dropbox/My Mac (computer name)/Documents

In theory, this is great, because you have one version of those folders that's always synced across devices. However, if for some reason you re-install dropbox, unlink your account, etc, and forget to turn off computer backup first, your user folders for documents, downloads, music, etc get really messed up because the OS is looking for them in a place they no longer exist, because the symlinks are now broken. I've learned this the hard way, very long story.

If dropbox backup worked like ChronoSync does, where it monitored a specific folder for changes without changing it's location or moving files around etc, and synced accordingly, it would be a much more robust solution. If you're looking for a set it and forget it, always mirrored folder structure, then dropbox backup will likely work very well for you. But if you have any kind of complex folder/file exclusion rules, or other parameters you'd like to set up for syncing, then you'll need something like ChronoSync to help you manage that. If you choose to go the third party sync root, particularly with ChronoSync, for now like mentioned above, both source and destination folders need to be locally stored somewhere, even if within dropbox, for ChronoSync to function correctly.
Sorry for the lengthy post, just trying to explain as much as I can in a way that makes sense and gives context. If you need/want to give me more specific examples of what your end goal is, I can try and point you in the right direction. Bottom line, what do you want to sync with dropbox, and how do you want that data to be maintained/managed across devices/backups? What do you want ChronoSync to help you achieve that you can or can't do with Dropbox backup on its own?