What’s nice about Syncplicity is that it allows a single user the ability to sync Google Docs with files in a local directory. In addition, you can install a syncplicity agent on every machine you use. Sounds like Dropbox, eh? Well, Dropbox has better sharing features, but you can still share documents via Google Docs sharing feature.
So if you want Google Docs to be your main repository of files, esp. if your company has just migrated to the Google Apps cloud, then Syncplicity is for you. For the rest who don’t need synchronization with Google Docs, Dropbox still works great.
Your list of “what’s nice” leaves most features unmentioned.
Syncplicity’s capabilities go FAR beyond that of Dropbox. The question is what are you trying to accomplish? If you are a single user, who has multiple devices that you want to syncronize a few files between, then I would chose Dropbox.
If on the other hand, you have multiple users (think 2 – 2,000,000 people) distributed anywhere in the world, then Syncplicity is infinitely better suited to the task.
What do I use? I use both actually, for the reasons above. I have multiple computers, and as a “Cloud USB drive” I use Dropbox, simple, easy and effective. I also work in a small business, with people in different locations. For this purpose Syncplicity works incredibly well, far better than VPN or WAN accelerators and a central file server. Dropbox cannot support this type of multi-user and muti-folder workflow.
And if you want to transparently sync from selected folders on a mapped drive that is mapped to NAS (network attached storage) on you LAN, you need Syncplicity, not Dropbox and not SugarSync.