Oh well this was an interesting one. I got no video output, thinking it was the SD card, I re flashed 3 different SD's but still no output. the Blue heartbeat indicate was pulsing away suggestion that all was well but nothing.
A post on the Hard kernel forum got a reply from the admins, to check the cable which I had done already, but then I noticed that when plugging in I did seem to get a slight signal capture, so I decided to fiddle about with the connector...sure enough, if I got just the right spot I got a signal.. but the slightest shake or vibration and the signal dropped. So it has a faulty connection.
I did manage to get it to sit stable on the desk for some time though to look through it, and it all seems to be fine and dandy, no disk volume size utility immediately obvious. no working OpenGLES demos on this one, but there was GLX demos, no source....otherwise pretty much the same as the XU4.
But I did note that it seems a lot more nippy than its big brother the XU4, giving me more suspicions that the X4U does not have accelerated GPU functions in Ubuntu
Sadly the vibration caused the signal to drop again and getting to stay stable was just too much effort, it has to go back. I've contacted Hardkernel and will send it off, or try to arrange for a 2nd one at some discount to avoid paying lots of shipping.