Up until recently I'd have said "avoid", but I've now replaced the useless BR-IM50 brakes that came on my bike with the newer BR-IM70 units. These have a much bigger cooling fin, and seem to work much better on steep Sheffield Hills.
As Mickle says, they're heavy and can be a pain to remove. In addition they need filling up with a special grease every so often, and the front hub has a "power modulator" fitted which is meant to prevent you from locking the wheel under heavy braking (and throwing yourself over the handlebars), but in reality just limits the maximum strength of the brake. And they rattle. And Sheldon Brown did not like them.
Apart from that I quite like them though - overall they do require less maintenance than other types of brake, work just as well in the wet as in the dry, create zero mess with brake dust, and don't wear out the rim! Probably better overall though if you live in a relatively flat area.