It is a 2-in-1 device: a tablet PC and smartphone. The phone provides the heart (meaning, CPU and RAM) for the tablet - which is not a real tablet then, but it "only" provides a bigger screen, ports, more battery, speakers and maybe something else.
The tablet has a docking slot at the back into which the phone can be inserted making the two devices as one. With this solution, there is obviously no data synchronization problem.
The PadFone is interesting, at least for me: I often I have the inspiration to write or to do other things in places like on the bus, but I cannot do it with a phone; a tablet would work. But on the other hand I'm not so willing to spend money for both a smartphone and a tablet PC: this Asus device seems to fulfill both needs at once.
So I can keep a light & handy tablet in my backpack and a phone in my pocket, and combine them if required.
Now, if Asus combined the PadFone and its own Transformer concept, they would have something with a very high degree of coolness.
Anyway, the PadFone is scheduled to enter the market at the end of this year, so its specifications and appearance may change considerably from now.