These are cool looking, but to me, it makes more sense to approach the solar powered car in 2 steps:
Step 1: Mount solar panels on your home's roof (if you own one!), and connect them to the grid. This will help power your home as well, whereas the car-mounted panels will not. Also, these panels will be optimized for sun angle and amount of sunlight every day, rather than the hit-or-miss of where you drive and park your car.
Step 2: Get a plug-in conversion kit for your Prius, to increase the battery capacity and to power it from the grid as much as possible.
While it is true that the exact electrons going from your solar panel are not necessarily the ones powering your Prius, common sense (and science!) tells us it doesn't matter. If I generate 300 kWh of energy from my panels, and my Prius pulls 300 kWh from the grid to get me around town, then I have indeed powered my Prius from my solar panels in every meaningful description of the process.
Another advantage is that when I don't drive, I'm saving up even more power for when I do, using the power grid as essentially a massive battery, which my solar panels incidentally just made a little cleaner.
The disadvantage is that those roof-mounted puppies might cost you a lot more, although perhaps the per-watt cost might be the same (since it's probably easier than the car-mounted installation). Haven't priced it yet, so not sure.
Another disadvantage is your car doesn't have a cool looking solar roof, and those house-mounted panels won't help you if you run out of gas in a nice sunny desert
BTW, I'm going to do this just as soon as I can convince my wife.