Finally finished this series last night; A poignant and powerful ending to a very strong show. It had its faults, certainly, but was absolutely worth the investment. One of the rare shows that respects it's audience enough not to spoon feed every detail. Cinematography, acting, music and direction were all handled beautifully throughout the series.
I can understand why the show didn't grab a huge audience, as it's not the most accessible series out there, but I can highly recommend that if you give it the time, the payoff and the way in which all the themes through the 5 seasons tie together is pitch perfect and very satisfying.
I would rank this series just below Breaking Bad and Game of Thrones on my list of favourite shows.