He wasn't all cool with their take on it (see below), but I agree that doesn't mean Martin will be able to end it in a satisfying way either. His last two books weren't as good as the two before it. He's gotten himself quite a pickle, but I have more faith in his ability than the HBO writers. We'll see (or maybe we won't).
I can't think of a single super popular series that ended with universal praise. It just doesn't really happen anymore, and that's OK. I'm more concerned with my own opinion when dealing with entertainment.
Martin in his own words:
George R.R. Martin criticises what HBO has done with Game of Thrones | JOE.co.uk
In a recent interview, Martin himself broached this subject, and admitted that he too is slightly disappointed with the way his story has worked out on television.
"Of course you have an emotional reaction. I mean, would I prefer they do it exactly the way I did it? Sure," said Martin in the interview with Rolling Stone.
"It can also be traumatic. Because sometimes their creative vision and your creative vision don't match, and you get the famous creative differences thing — that leads to a lot of conflict."
"You get totally extraneous things like the studio or the network weighing in, and they have some particular thing that has nothing to do with story, but relates to 'Well this character has a very high Q Rating so let's give him a lot more stuff to do'."
Martin said: "The series has been... not completely faithful. Otherwise, it would have to run another five seasons."
In another interview with Fast Company he added: "You know, it’s complex. I’m a little sad, actually. I wish we had a few more seasons."