I've watched 2/3 of the first series of Game of Thrones, after reading and enjoying all the books. (Critique of problematic aspects to follow.)
With many adaptions, the best you can hope for is that the new version doesn't lift the most interesting scenes and character moments from the original, without any of the background that makes them make sense. (Woo, you found the macguffin. That would be really moving if you'd ever mentioned the macguffin before...)
But a really great adaption is one that adds to the origianl, expanding themes that are in keeping with the original, but bringing them to life. I think this is different than "better or worse" because an adaption might cut down an original a lot, but expand it in some ways.
I can't think of many examples, but there are more than you might think. Some would be:
* Tie-in novels written by a good author, that are very good in their own right
* Films based on forgettable books
* Douglas Adams constantly adding new material in each version of Hitchhikers
I feel game of thrones falls into this category. I didn't find it better than the books -- it missed a lot of internal explanation of the characters that's hard to put on screen. But it definitely added to the experience, with the visuals (including the opening credits), the portrayals of the characters, bringing to the fore some aspects of characters which only really become specified later, etc.
For instance, the rampant sex and nudity, while sometimes innapropriate, often does a very good job of showing whcih characters get quite debauched, which we only really heard about in the books.