I will say that the Kinvaras are more squishy and less responsive then most shoes. They don't work well for races. As such, I got the Saucony Fastwitch 5...7.8 ounces, 4mm, and very responsive. There is a small medial post for mild pronation when you get tired on really long runs. I'm planning on running Boston in them.
I had a pair of Newtons last year. I found them somewhat flimsy on the trails & geared for narrow feet. They were inferior to NB trail shoes and not nearly as comfortable or durable as Altras.
Since there has been so much discussion of Altras, let me add this. The Altra Instinct (older version) is in my current rotation. It's a zero drop shoe with a very wide toe box and no arch support posts or pronation devices. It's a relatively stiff shoe, far stiffer than minimalist shoes and while it has more cushioning than a racing flat, it's firm cushioning. It's also heavier than most racing flats. In 9.5, mine weigh a little over 10 ounces.
To answer the OP's question, there is almost no similarity between the Kinvara and the Altra Instinct except for the low heel (4mm on the Kinvara and zero on the Altra). The Kinvara feels light and squishy. After 8 miles, my feet begin to feel sore in the Kinvaras. On a track, I prefer the Altras. They feel hard underfoot, while the Kinvara feels too squishy.
While you can put on the Kinvaras and run with them right away, the Altras take some conditioning to get used to the zero drop. It feels A LOT different. Heel striking is painful, so you quickly adapt to landing forefoot. Calf muscles will be sore until you adapt to zero drop. The Altra is not a minimalist shoe nor is it a traditional shoe. It is something altogether different. At faster speeds, it felt more comfortable than at slower speeds. In my case, "fast" is sub 7:30 and slow is over 9:30 per mile. If I did all my runs at sub 7:30, I'd probably like the Instinct more. All the information here applies to the old Altra Instinct, not the Altra Instinct 1.5.
By the way, Altra has a new shoe for 2013 that has a thicker insole, but retains the zero drop.
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