Biggest downside that I see is that you have to carry a laptop and a tablet dock with you... with the Padfone you only carry a netbook, which also happens to be able to transform in a tablet And Ubuntu, I know it, and probably most of the people reading this know it... But Android is waaaay more mainstream and used by every "non-technical" customer, if they're going to buy a device like this (nexPhone or Padfone), the Padfoen will win hands down..
I do think that they are going to fail (let's face it mobile phone market is not driven by quality, but by name - that's by Apple is still around) I don't mind separate tablet and laptop (altough it would be nice to have PF1 style - we all know it is possible) I also think that Ubuntu is for me (I use Linux Mint on my PC and I have no software knowledge what so ever) and changing it to android should be easy (if it's ubuntu than everything is open and accessible). What throws me off is connection between the phone and tablet/laptop - it's just ugly they could just as well do it on cable (back camera is unusable the way they did it, it also doesn't look safe but so does mounting it PF2... but I would rather trust ASUS).
looks kinda ugly but i do love all the devices it can plug into with ubuntu for desktops ... i hope asus release some more devices for my phone to dock into
There are a lot of things that look promising, even inspiring, on paper - remember the "holo keyboard" which was set to revolutionise input, but which has now been completely forgotten?
Designers have a particular knack for making things look desirable, and I've no doubt there'd be people out there with money to burn but no real need to use the technology, who'll buy it just so they can show off - but in the everyday world, the execution is technically wrong.
As LukaszMM mentions, they haven't bothered to take Asus's idea of re-using a perfectly functional main camera - but this is just a minor irritation compared to the other conceptual flaws.
I can see the docking connectors on the back of the tablet and laptop docks getting damaged very quickly, I can see all of the devices developing chips and nicks very quickly (unless they're planning on using titanium as the building material), and I can see people losing their minds if their 'phone gets lost or stolen or needs to go in for repair, as absolutely all of their data is just on one (very volatile) device.