Martin kind of wrote himself in a corner (by his own admission). I would guess that the HBO contract included some varient of "you must finish the books" which is why he is getting back to it by now.
He's a great writer - probably the most natural writer of the old big three. I do have to say that the new generation of fantasy writers - In particular Patrick Rothfuss's unbeliavably great series The Name of the Wind, and Brandon Sanderson (who took over the Wheel of Time books from Jordan) are better authors I think then any of the three.
(If you haven't read The Name of the Wind, you need to. It's Tolkein and C.S. Lewis good).