Hopefully the clue was in the title of the last episode, end of the line.
Earth invasion shows are my thing, so elements of both series 1-2 kept the interest going despite the laughable acts of most of the story arcs. Series 3 was actually pretty tight and a lot more formulaic, aka watchable. The last episode was pure tripe though, seemed spliced together, with a 'and that's it' ending.
Fingers crossed it doesn't return for a forth series.