I got the Samyang 14mm f/2.8 in mid 2012, as a former owner of the legendary Nikon 14-24/2.8 I was amazed that Samyang could make a lens performing almost as good for that kind of money.
Here's a mugshot on the A7R, coupled with a combination of two Noveflex adapter rings (a tripod mount can be attached to one of the adapter rings):
Its ultra wide angle FOV, fast aperture and excellent sharpness wide-open makes it a very good candidate for shooting startrails. One gripe is the lack of a hard infinity stop while not being able to disengage the focus ring (focus by wire does have its advantages) so the trick here is to bring gaffer tape and fix the focus ring once infinity is dialled in to prevent the focus to be shifted accidentally.
Startrails shot somewhere in the snowy Spanish Pyrenees (D800E, 20 minute exposure, f/5.6, ISO200)
And two shot on the Norwegian / Swedish border during a two week canoeing trip in september 2013, my last trip with the D800E before I jumped ship to Sony (half hour @ f/4 and ISO100)
Here's two from recent NG trip to Slovenia, shot on the Sony a7S: