I was really late to this and just watched 3 series back to back.
I've really enjoyed it as some pretty dumb sci-fi TV, and for a while it looked to have some real promise - but a few episodes into S3 it was starting to feel really drawn out and lacking in direction. A lot of the individual episodes are definitely getting weaker.
Will definitely watch S4, but fully expecting this to follow the standard model for 'successful' US TV - commission short first series, wait to see if popular, increase budget, dilute storyline to drag out show as long as possible, recommission new series until viewing figures drop, wrap up show with weak ending.