6 is definitely worth buying. A lot of good races(rainmasters was my favorite), finally added Mount Panorama to the GT series, Goodwin festivals were fun, physics are a bit better, top Gear track is gone unfortunately, no silly B-spec crap, overall a lot more fun to play than 5 was.
Mount Panorama is my favorite new track. Nurburgring Nordschleife will always be #1 though.
I actually played GT5 today for the first time in a year. I forgot how therapeutic beating on virtual cars can be lol