Because you own the hardware, the data remains in your custody; there are none of the awkward legal issues about transferring confidential or copyright content to a cloud provider. Because you own the hardware, there are no on-going costs (except for about 3W/unit of power), and thus no risk of sudden price rises.
Although the data transfer between Transporters happens in a peer-to-peer network, the peers depend on a central rendezvous service to locate each other and manage the network. In the light of the current Google Reader fiasco, I asked Geoff what happens if Connected Data goes away or loses interest in the Transporter, so this service stops working. He replied that in that case they could open-source their central service software, so a replacement for Connected Data's service could be provided.
I'm initially using my pair of Transporters from Linux, so I haven't yet used their desktop software, which is for Windows and Mac. It makes things even easier by transparently sync-ing local copies of files and folders with the Transporter. Despite this, it only took me a couple of hours to unbox, load disks into, bring up, set up accounts for, and start using my two-node network. One of my Ubuntu machines mounts one Transporter via SMB/CIFS and various cron jobs use rsync to ensure that new or modified data gets there. That Transporter syncs automatically to the other; I will shortly move it to a remote location so as to provide off-site backup.
I could, of course, have hand-crafted something like this using small Linux boxes such as the Raspberry Pi, the same disks, and a cobbled-together Web interface to manage it. I might have saved a little money, but it would have taken me a long time and it wouldn't have worked as well. Especially not for the other family members who will host my off-site Transporter, in return for my hosting their off-site backup copies.
The Register has an informative pod-cast interview with Geoff, including how they used Kickstarter, but he only starts to speak after 14 minutes of self-indulgent BS among the hosts, which you should definitely skip.