The short answer is that its a bit of a mixed bag, and whilst we were watching it, it took me a while to work out why. It wasn't really until "The Mountain and the Viper" that it clicked, but then I had the answer; its a show at war with the narrative that it's adapting, and I think the strain is starting to show. I'll come back to that in a minute, because its probably better to start with what is great about the series. Firstly, it's lavish to watch; importantly for a show set in a fantasy world created without any sort of budgetary constraint. The world looks and feels real, the locations are consistent, and there isn't really any of the "hey, its that quarry again!" that you often get. Immersion is key, and Game of Thrones really nails it.
Secondly, the casting remains top notch. There are a couple of exceptions, but mostly the cast leap off the page and in many cases can make a minor character leap out of the screen far more then you thought possible. It also adds nuance and depth to much of the non-POV characters that widens out the world, even as much is jettisoned due to the running time of the show vs the door-stop novel lengths. Finally, for all the problems I'm about to mention, the scripts and direction are sharp and do what they are being asked to do very effectively. But this brings us to the deeper problem, that gives me some fear for the future.
The problem with any adaption is so obvious we rarely mention it - that TV, film or text (or comics!) are all different mediums with different strengths and weaknesses. Moving from one to the other can sometimes expose these weaknesses, but also the strengths of say, television, can cause some problems, not least of which that the information that visual medium can convey is very different from text. In the first series this sort of manifested around Tyrion, who in the books sees himself as caustically witty and clever, but as this is all from an internal perspective, he could equally have been misjudged, arrogant and cruel. The TV show had to make a call on that, and thankfully we got the unstoppable awesomeness of Peter Dinkladge's performance, but you see the point.
In season four this manifests mostly around Shae. Back in season three, Shae and Tyrions's relationship is presented as much more nuanced and mutual than the book made clear, because mostly we are seeing it through Tyrions uncertain eyes. But again, the TV has to make a call on it, and the scripts played both parties very sympathetically, and let her build a relationship with Sansa along the way. Once we move into season four however, Shae's destiny is foretold, and her behavior is foretold, and suddenly her character seems to change to place where she needs to be and that complex, interesting and sad relationship is lost. Jon and Ygritte suffer a similar fate; whilst they were never going to be happy, she's a character that benefitted hugely from the jump to TV and she's pretty brutally dropped from the narrative before popping back up at the end to get fridged.
At it's best, Game of Thrones has been a show that has taken its multiple strands and, within and episode, strung them together to make thematic points even though the characters rarely interact. Increasingly, however, it can't seem to manage that, an episodes become almost like a sketch-show, dropping in on locations around Westeros with little sense of interaction between strands. And this feels driven increasingly because characters have to be at certain places at certain times for the future, rather than following the arcs that a show without this constraint would allow. Some of its strongest scenes seem to be invented for TV, changes that bring strands together directly, or give supporting players chance to shine.
Look, Game of Thrones is far from a bad show. It's a pretty great one. But in an episode like "Mockingbird", or the aforementioned "The Mountain and the Viper", you get an hour television with a big climactic event and pretty much nothing that happens in the preceding run relates to it, giving it a strange, fragmented feel that probably lends itself to binge watching. There is a fear that, unable to be free to let the reworked characters and stories run where they may, the producers instead are leaning on having a "shock moment" per episode, which no matter how well done it is, twists the narrative flow around. Despite this, I remain a fan of the show, and, knowing what is to come for many of the characters, await season five with interest.