Exceptional. It's an amazing show. It's a little pretentious at times, but the casting for the show is excellent, the character development is outstanding and the plots are brilliant. Series one was great, series two was better than the first, although it did dip in places due to the huge storylines running through the show. I initially thought Series 3 started very poorly compared to the previous two seasons but it has built up to be equally as good, if not better. I actually love the start of Season 3 now because it covered so many gaps and was mainly building the plot for the rest of the season.
The musical score to the show is sublime, some amazing music throughout each episode and it builds tension and suspense and accompanies the show perfectly. The dialogue is crisp and intelligent and the whole show is exceedingly well polished. It's hard viewing at times because the subject matter is so dark, but that makes it even more compulsive viewing in my opinion.
If you like the books or the films or have any interest in them at all then I would say this show is unmissable. If you haven't read the books or watched any of the films then this is the perfect place to start. It's easily a 9/10 show and personally, I think it's the best thing on telly at the moment.