The 6th book is already completed, Winds of Winter, and Martin recently said he's happy with his progress for the 7th book- A Dream of Spring.
He's sky rocketed in popularity and probably let a good amount of that fame get to his head. All the little things he's been able to partake in because of the TV show, I don't blame him for coasting. That said, my conspiracy theory is that he's waiting for the tv show to end before publishing his last books. That's so he has the definitive last word on any story narrative.
I recommend the books, #obvi. Martin's writing is consistently wonderful through the series. The books are more in depth, and have more surprises. The nuance is much richer than the tv show.
Wheel of Time... I finished that series, it was worth it in the end. Sanderson did a good job of filling in for Jordan, and while there were some tonal differences, I appreciated Sanderson not writing for a half a page about the color of trim and transition of fabric types when describing a dress. If you remember the story, it is worth finishing.
I concur with not starting a series until it is finished. The Sword of Truth got me, I hate read so much of that, just to finish it out.