I've had both the Grizzly 660 without power steering, and the Grizzly 700 with power steering. I'm not sure the power steering version of the Grizzly 700 is all that big of an improvement over the non power steering version of the Grizzly 700, because I haven't rode a non power steering version of the Grizzly 700. But I am sure that a Grizzly 700 with power steering is a huge improvement over the Grizzly 660. The whole machine feels much more balanced, feels more sure footed and stable, and has significantly better handling. It is smoother, quieter, and more responsive.
We bought a 2007 Grizzly 700 with EPS in August of 2006, and really liked that ATV. At the time I had a 2006 YFZ 450, and the Grizzly 700 was for my wife. But I rode her Grizzly as much or more than I rode my YFZ, so one year later we sold the YFZ. At that time I could have bought any big bore utility ATV I wanted, and I looked at Can-Am Outlander 800s and Renegade 800s, and the Brute Force 750, and test rode all of them at local dealers. In the end I bought another 2007 Grizzly 700 with EPS for myself. The only difference is that mine is the special edition silver Grizzly, so its faster than my wife's camo Grizzly. I was impressed enough with the handling, steering, and overall great terrainability of the Grizzly 700 with EPS that I bought another one.