The first few episodes are very enjoyable, but there is a moment around the halfway point of the season where it jumps the proverbial shark (the scene with the tail, you will know it when you see it).
The latter half of the season then sheds most of the earlier mystery, which meant that all we had left at seasons end was Tatiana Maslany's (admittedly very excellent) performance.
I am trying to think of another show in which an actor so convincingly portrays four very different and distinct personalities. At times you forget that it is the same actress who is playing Sarah, Alison, Cosima and Helena.