I ordered both the GTS 10 and the GTS 11, having moved up from GTS 7 and GTS 8. I skipped the GTS 9 line due to not having liked how it fit my feet when I tried it on at a running store.
There really isn't a lot of bad things I can say about the shoe, so I'll compare it with the GTS 7, 8, 10, and 11.
About me:
I'm 5'8" 165lbs, and medium distance runner (6-9 miles or 45 minutes to an hour), no to mild pronation, low to medium arch, narrow to medium feet width.
Comfort: The GTS 10 really feels the most comfortable, and is a bit like how the GTS 8 fit around my feet. The GTS 11 feels a lot like the GTS 7 in terms of comfort. They are all really comfortable, but as far as fitting goes, the GTS 10 really molds around my feet perfectly, but a little more so than the GTS 8.
Cushion: The GTS 10 provides the best cushion for the balls of my feet, the most solid arch support, and heel cushioning. The GTS 11 felt a little low in the arch department, and didn't have as much cushioning at the heels. It's a little lighter, so I feel as though I could run faster, but doesn't feel as good when keeping a steady pace.
Durability: The GTS 7 and GTS 8 both lasted a good solid 1-2 years, replacing the insoles every couple of months. I've yet to see how the GTS 10 and GTS 11 fare.
After-run thoughts: My most notable pain I feel when running is not in my knees or ankles, but my shins. The GTS 7, 8, and 11 just don't give me the cushioning I need to ease the pain from my shins (I have off and on shin splints). The GTS 10 seems to take the pain away from my shins, and I really feel great after a run!
That's it for my brief review. In short, the GTS 10 feels like a better upgrade over the GTS 8, and the GTS 11 feels like a better upgrade over the GTS 7. Everyone's body and feet and running behavior is different, but if you somewhat resemble my stature, running length/speed, and feet type, I highly recommend the GTS 10.