Differences between Harb Carvers and Streetskis
First, I'll answer the question about comparison to inline skate type devices like the street ski (and I'll asnwer the Pro vs. comp question in a few minutes).
I have not tried the streetski. However, a few racers that have bought carvers from me had tried street skis before they got carvers. There are a few things that I can say based on their inline design.
In the last International congress on skiing and science, someone from the Austrian national team gave a presentation on the limitations of inline skates for ski training. Their conclusions were consistent with the experiences that led Harald to develop the Harb Carvers.
One of the biggest problems with an inline wheel design is that the hinge point for tipping is directly beneath the foot, so the skier must actively keep the skates from tipping.
However, as John pointed out, skis are the opposite from skates in this regard.
Because skis have some width, they actively resist tipping. This is also why they have some stability for both general balance and also at speed. They want to retore to a flat state and the skier must tip them on edge with the hinge point being outside the width of the boot.
This also makes balancing different. Balancing on an edge offset from the center of the foot is different than in a skate. The contact point for balancing on a tipped skate is directly under the foot -- nice but not at all like balancing on the outer edge of a ski.
By using two rows of wheels on each skate, Harb carvers simulate the same flat and edge to edge feel and movement requirements that skis have. Another by product of this design is that the wheels hold on a much higher angle - some of this has to do with the way the wheels deform on edge and other things. Also, the relationship of how the grip of the wheels is related to body movements is the same as on a carving ski rather than on a skate.
At ICSS, pictures of the best Austrian skiers trying to lay skates over as best as they could just didn't even come close to what is done on skis or on carvers. They dropped their jaws at how layed over people in Carver videos were.
See the pics and vids here for examples:http://web.pdx.edu/~petersj/HoodCamp...rvingIndex.htm
Another difference that I suspect, but don't have any direct experience about, is the role of skidding/sliding with the Streetski. The Streetski has longer, flexible frame design, as well as, wheels under the foot to act as a pivot. In several spots in their literature there are references to using wedge type movements to slow down, sliding the rear wheels, pivoting, etc.
These are aspects of skis that the Harb Carver purposely do not attempt to simulate and it is what makes them so effective for learning. Skis allow you to be sloppier with your technique. They will forgive an enormous amount of steering, twisting, and rotation and it is harder to feel the presence of these movements on skis. Harb carvers do not respond effectively to steering or rotating. Everthing must be done by lateral tipping, independent leg flexion for weight transfer and CM movement, and complimenting upper body movements.
Harb Carvers give constant and sensitive feedback on these two different sets of movements. Rotory movements (even in very small amounts) produce negative results and lateral movements prodeuce very positive results.
For those people who are trying to do PMTS movements, work the carving parts of their race technique, Harb Carvers give great feedback and many people say they learn more about thier skiing in a month on Carvers than a season on snow.
For those who want to hang on to steering movements, I would say that Harb Carvers will open up a whole new sensitivity and control of lateral versus rotary movements. Those people will greatly improve their lateral movement skills in their total toolbox and wil be able to develop more fine sensitivity for the presence of rotory and steering. If you don't think that lateral is everything, that's fine. Use the Carvers to sharpen the lateral tools in your box and work the other stuff on snow.