I have a friend who has heated his shop with one for a couple years. Seems to be holding up well. I don't know if I would use one in a house or cabin though. They take up a lot more floor space than a small wood stove.
If you buy a kit with the barrel included, they usually come coated with a black heat resistant paint, though as all metal things, it will decay in time. They do last for many years though, the Barrels that come with the kit are typically made specifically for the purpose these days (cheaper to mass produce than buy from another source) and have a thicker wall than say, oil drums. Ive had one in the shop for about 9 years now and still going strong. In tim any rust should be buffed out with some steel wool, and wipe the dust off with a damp rag, after it dries, repaint with some high temp (such as engine block or grill) paint.
From what others are saying, it seems like they last pretty well. Just remember that if a barrel does burn through some day, that they are easily available and always will be. All it takes is cutting out new holes and installing it. If you find a couple freebies somewhere, you might put them back for such a day. I keep a bunch of barrels around to store fuel in and always seem to have a couple empties.
We had a barrel stove in the basement when I was a kid. We put a layer of sand in the bottom to keep the barrel from burning through. We also tacked 2 pieces of angle iron on the top and laid a steel plate over them. That way we could keep an old coffee can of water on it for humidity, but it didnt boil off so quick by adding the irons and the plate...sure heated nice!!!