We tried one and it did make things "better" but the ultimate cheapy solution for us was a tractor heater. It has a big magnet and sticks to the oil pan. We did put a strap on it so that it wouldn't drag along on the road under the car if it were to come off of the pan, but I don't think that it ever did. That was just as effective in N. VA winters as the block heater was on the other 240D and the 300TD. The 300Dt would start here even if it wasn't plugged in, although we did use the block heater to lessen the strain on the battery.

-Tad