not very practical. if your playing it, that side will be facing down. and if ur not using it, u won't exactly be leaving it outside in the sun would u? also, if ur playin oustide, i guess you could lie down but then the sun would be in ur face
if you could fold it so it's on top then it would be usefull when playing, otherwise no, not really, but very good that people are making stuff like this, just has to be refined and then it's going to be very usefull
and, it looks rather dangerous, you should never connect a voltage source directly to a lithium ion battery, it could cause it to explode. and whilst the meagre amount of power from a solar panel may not be enough to cause that, it is highly likely that it will cause the battery to over charge and become damaged over time.
it would be a lot better to solder the wires from the solar panel to the power input pins. It is likely the regulators inside the DS can take the voltage variance.