sure you can get high voltages from them... but think of the amount of shock that it takes to get that, with the spring mechanism, it smacks the crystal pretty hard. you would probably get a much lower voltage if you had it in your shoes, or whatever.
also bear in mind that they only produce electricity when they are in the process of being squeezed, not when they are held rigidly in place, compressed. so if you stuck one in a vise, you would get some electricity while you cranked it closed, but none if you left it in place compressing the crystal.
also bear in mind that voltage is not the only factor here; crystals surely produce an incredibly small current, and since power is the product of voltage and current, the actual POWER output would be incredibly small as well.
I believe there are watches that use piezo elements to keep their batteries charged... however bear in mind that watches use an insanely small amount of power to begin with... even using a much more powerful array of piezo elements, i think charging a standard-capacity battery would be rather impractical