Just finished watching it and, like some, I´m a bit on the fence. It´s a good show that has a lot going for it, most of all the cast. The acting is outstanding at times and gives you some really beautiful moments between the characters. John Nobles performance is among the best I´ve ever seen on TV. And I became very fond of Anna Torv. And despite the fact that a lot of the scientific background is borderline at best the show never feels silly.
But like some others mentioned I too think the plot went south during the later parts. The first two season got me hooked pretty fast, the third I think was at times to much to fast. But with the forth came the nosedive. It deteriorated to a point where I simply didn´t cared anymore for the plot and just went on watching to see how it ends. Not to mention that the conclusion has huge logical problems.
I´m not regretting watching it but I don't want to watch it again.