This, This and This. Books 1 to 3 were magnificent, but 4 was atrocious and the rest mediocre. He created a good and recognisable fantasy world but is nowhere near the best fantasy writer of our time if you take consistency into account.
David Gemmell for example was a class above imo.
I like Gemmell a lot, but Martin is just more my thing. He nails that sense of scope like no one else imo and I much prefer his style of writing.
All this discussion just proves that it really is a subjective thing. 4 and 5 are my absolute favourite of the series by a country mile, though I can totally see why some perceive them to be his worst what with so many new characters and plot threads being introduced.