A number of things, The clasp was incredibly stiff. The first time I showed it to my girlfriend she put her hand on it whilst it was on my wrist and got her finger very slightly behind the clasp lever and the watch came clean off. The bezel didn't line up properly and there was a degree of vertical play in it which made it feel very cheap. From a more subjective point of view I don't like 60 click bezels.
By all means get one second hand, or possible play with one - like Oris most people are more in praise of the brand than I - but I feel fairly sure if you held the U1 next to the Marine Master there would be no contest which is the high quality piece.
Also not all Sinn U1s are necessarily like mine, I think the quality control has dipped somewhat over the past few years, but buying mine new after saving for two years and it not being anything like the perfect quality product I was expecting left a sour taste. That said the U1 is a great looker and despite my experiences, when I see them on the various sales fora for Â£600 to Â£650 I am tempted.