I mentioned somewhere else (may have been a different website) that I've heard 25th Hour was excellent, so it's entirely possible he's good at writing grounded realistic stories and just bad at writing anything in the fantasy/sci-fi realm. X-Men Origins: Wolverine is a train wreck in every way imaginable, and Troy was a massive disappointment.
Game of Thrones definitely wasn't getting better as it went along. It was becoming a bigger spectacle with larger set pieces, which may be all some people wanted, but the writing quality got progressively worse. The first 3 or 4 seasons are easily the high point of the series from a writing standpoint; it goes downhill quickly after season 5.
It absolutely was hugely successful, but I think at this point it's pretty clear the success was almost entirely due to George R.R. Martin (Benioff and Weiss still had to do a good job adapting the early seasons, so Martin shouldn't get ALL the credit). That's been bothering me for a while, though (and not just with regards to Game of Thrones) -- for some reason a lot of people in the TV/movie world, critics included, seem to ignore the fact that source material exists and give all the credit to the people writing the adaptation as though it was their own original idea. There were whole articles written about awesome decisions Benioff/Weiss made, how they were subverting expectations, etc. that completely ignored the fact that the things being praised were pulled essentially verbatim (sometimes the dialogue was actually verbatim) from the novels. I never understood why they were getting credit for that.