I just watched the whole Show, from S1-S5 over the Thanksgiving weekend, and enjoyed it all. S1-S2 is a story arc of it's own, S3 advances that Story Arc, and brings it to a head, and then S4-S5 is another seperate Story Arc, that advances what came before.
S3, perhaps should've been a little shorter like the otehr 4 Series, but, overall, I really enjoy the whole Show. Looking forward to the Canadian Spinoff and what they do with it, since S5 ended so well, and Canada is going to do their own, I am assuming the existing show won't be continued any further. Nice wrap up, IMHO.
Shame about how often the cast changed, but, I think I really enjoyed most of the characters. Never really got into Stephen, and Sarah didn't really contribute much, but, everyone else was pretty good, IMHO.