I've got a Minn-Kota Endura 42 that I hang off the back of my drift boat. I use it primarily to position the boat when drift fishing or casting spinners. It works quite well, just run it back upstream and you can put the boat pretty much right where you want it and don't have to deal with constantly messing with your anchor or sitting on the oars while your buddy fishes. Its also very useful when fishing by yourself.

In smaller rivers or holes with swirling currents its usually better to just throw out the anchor. On the negative side, once you get the motor and the battery in, your drift boat becomes rather tail heavy.