Seasons 1-3 are the heart and soul,
This review is from: Being Human (U.K.) Season 4 (Amazon Video)
When you lose John Mitchell, Geoge and Nina, I lost interest. As a completist, I came back and finished the Season. Good writing, good episodes, but you miss the departed characters from previous seasons.
It would've been interesting where they took the story vs the British X-Files like enemy in Season 5, but without Annie, it would've been also strange not having an original cast member present.