To contrast the medical definition of a keloid, here are my personal experiences with them:
They hurt. But not in a constant way which you should take painkillers for. For a while, they're fine. You can see a large growth but aside from how it looks it just doesn't bother you. Then after a few months or so, it starts to itch occasionally. Maybe a few times a day. Then after a while of that, it gets past the itching stage and starts to hurt. This is simply because it's stretching your skin. Skin's really not meant to be stretched that much. I've personally noticed the large keloid in my left ear hurts much more in cold weather too, much more than my keloid-free right ear, which is why I tend to wear a hat a lot.
They only grow in certain places. I don't know if this is the same for everyone or not. I've had a rather bad keloid on my left ear for as long as I can remember, and I've had at least four or five operations to remove it so far. The problem is that the operations themselves can cause further scarring and the whole process repeats. But since I got my main keloid (I didn't know what it was at the time) I decided to get some piercings. All of them were fine, with no problems at all whatsoever, except for my earlobes, which then grew comparatively very small keloids. I took the rings out and they've stopped growing, though they're not going away either. The main one will probably be back though. So a good idea is not to get any piercings or tattoos where you're susceptible to keloids, or preferably not to get any at all.
They're not too hard to remove. The operation to remove a keloid usually doesn't last that long compared to major surgery, and you can often go home the same day you went into the hospital. Thanks to anaesthetics, you'll probably only feel pain while you're waiting to go home after the operation, especially if someone comes to tighten your bandage.
They'll probably be back. It's a good thing the operation's not too bad really, as you're likely to need another one, and another, at regular intervals. Thankfully these should be several years apart from each other.
So in conclusion, your body not knowing when to stop making a scar can be pretty annoying, but it's not the end of the world.