They can potentially be both a death trap and fun.
The ones I've found have huge amounts of coal and iron. Sometimes, they even have gold and diamond. They usually do connect to caves and sometimes dungeons as well. The only downsides I've found is that one slip and you fall to your death and they usually have dark areas where hostile mobs spawn.
I have a huge ravine near my my fort, its so long and huge that it even goes under a mountain range at one point.
The sheer cliffs look like a death trap... but possible with ladders, blocks ect... But wondering if these ravens are worth exploring or just staying the hell away from.
I like ravines. I've found 3 in my world so far. one on the surface that I set up a base camp in and 2 underground with a river of lava at the bottom instead of water. not the most efficient place to mine but certainly the most exciting.
Personally I think that ravines are great, because they give you good supplies such as iron and redstone without having to mine for ages. It's also good because it's not too easy to get to them as you have to climb down, and creature spawn because of the dim light.