Sounds cool, I'm not sure how much power one would make though (like you said, it would be small) and it could take quite a while for the device to pay for itself (cost of unit, electrical connections, installation). The installation could be quite pricey if you have to bury the device even 4 or 5 feet into the ground, though I guess if you were to make a field or array of them you could use an excavator for the digging. The good thing about it is that it seems like there would be almost no maintenance necessary (much less than solar or wind power) and they could potentially last a very long time without needing to be replaced. So once they are paid off everything else is a bonus.
Solar panels / wind turbines (I think) would produce much more power for the cost, but they do require a lot more maintenance (cleaning panels, wind turbines have moving parts that wear out) and have more limited life spans. You'd really have to sit down and crunch all the numbers for cost, projected power output over time, maintenance, life span, etc and see which one ends up being better in the long term.