The first and 1/2 of the second season were great, you really felt that there was an epic tale to be told (although there is a gap between the shaky formation of an alliance at the end of the first season and a full-blown civilisation at the start of the second).
From there, it goes downhill, descending into ridiculous action in an obvious ploy to gain ratings. Some episodes are really good, most are bad - leading to the departure of Tyr. He is replaced by Telemachus Rhade, a worthy replacement. It is a shame that the two could not meet in more onscreen encounters; it would have been interesting to see the two characters play off each other.
The end of the fourth season sums the show up; it tries to be mysterious, but ends up so bad you can't even laugh, you can only cringe. The fifth season has only one redeeming factor; it shows the acting talent of Steve Bacic, as his character is extremely different to how he was in previous seasons.
Overall, the first season or so would have gained a much higher rating, but it was brought down by the last few seasons. Great concept though.