According to Todd Day, the acknowledged expert on DSM ECUs, this is not a good idea for 1Gers in search of serious speed. He also stated there are at least 17 temperature-dependant tables in the ECU, so skewing the engine temperature might not be a good thing. However, many people have installed them with no ill effects.
It is possible that the lower temperature thermostat will operate ok in the summer but not in the winter. Some owners have commented that their datalogger temperatures stay in the 190 degree range in the summer when using a 180 degree thermostat. This is usually high enough to keep the car operating as intended. In the winter, though, the average temperature drops too low, and the car never exits "warm-up" mode. This causes the car to run rich and drop significant gas mileage.
2Gers are different. Apparantly, 2G cars have a 180 deg. thermostat from the factory, and at least one DSMer has installed a 170 degree one. However, the same caveats about lowering from the stock temperature apply equally well to 2G cars.