SOTS (Solar on the Surface) is not just for moving the car forward.
And here I thought the whole purpose of charging the Volt's battery
is for moving the car forward (or reverse)
There are many that want to tell you it's not cost effective or that it has no use. Please go through the treads and make up your own mind. Why do they hate solar so much? Why do they feel people only buy cars or select options based only on cost (are we all driving Geo Metros?) is beyond me.
I don't think anyone would say that the solar cell roof would be of no use. However, the cost of the options (or the vehicle itself) is of critical importance. For many, a $40k price tag is out of reach, or barely achievable. The "sunroof"

sounds like a worthwhile option, but it needs to make financial sense. Would I pay $200 more for it? Yes, absolutely. $2000? Not so certain.
I don't think anyone "hates" solar - it just needs to make sense. The active surface can be only so large, and that will generate only so much energy. I believe you will agree that by itself, the solar roof will provide insufficient energy to provide a meaningful charge. It's 500W (guess) output is better than nothing, but it's not like I can drive the car 40 miles on the battery, park it in the sun for an hour-long leisurely lunch, and drive another 40 miles without causing the ICE to run. Plus, it needs to be robust. What happens when massive hailstones (or the neighbor kid's b-ball) comes down on it?
I'm looking forward to see the solar roof - I want to see how well integrated it is. Just imagine something like a low-profile luggage rack with a big, one-meter square flat solar panel on it?
BTW, that SOTS boat looks pretty cool. Something like that could look good on the Volt. Can I get the SOTS in a different color? (This is a joke, and I would have put a smilie after it, but I had reached the limit of four already.)