I had a Sinn UX (the oil-filled, any-depth-capable, quartz version) for a while a couple of years ago, and that uses essentially the same case, bezel and bracelet as the U1. (The caseback is different though). They're nicely made watches, and I really liked the head - very industrial and tool looking with a good fit and finish - so I think they're very good value for money.

I also liked the look of the bracelet, but one thing that really irritated me was the fact that the bracelet wasn't hardened in the same way as the case. This also meant that the colour was very slightly different to the head. It's not a hugely noticeable difference, but once you know it's there, you will see it! (Sinn may have changed the bracelets now to be hardened like the case, but when I had mine they were just standard steel).

The other thing I found with the bracelet was that the bead-blasted finish is nigh-on impossible to refinish or touch-up yourself once you start getting scratches. The head has the same bead-blasted finished, but the hardened submarine steel is much harder to scratch, so this is less of a problem. Again, if Sinn have started using the same steel for the bracelet as the case this shouldn't be a problem.

The rubber strap option they have looks nice, but it's a dust and lint magnet, so if that sort of thing bothers you, I'd suggest sticking with the bracelet.

Other than that, the only thing to be aware of is that the service is done in Germany as has already been said. Obviously for a U1 (with a standard ETA automatic movement - I think it's a 2824) you could easily get it serviced by a competant watchmaker elsewhere if you wanted to avoid sending it to Germany (subject to losing the warranty, etc). However for the oil-filled UX I had, there was no option other than going back to Germany, and that was part of the reason I sold it.

All in all though, the "U" series are very good watches and good value for money IMO.