These units, once developed can be a devastating force to deal with when facing the Zerg in combat. While they aren't terribly tough even once their armor is totally upgraded, the fact that they can be created so quickly makes them one of the most powerful units in the game.
The process of obtaining them is tedious and time-consuming and people who decide to use them (and you should) need to focus on getting their greater spire developed as soon as possible.
The only way to get Broodlords is to make Corruptors and then incubate them for a time until they re-emerge as what I like to call the siege unit of the sky.
The only problems with Broodlords is that they take a long time to develop (relatively) are totally defenseless against anything in the sky and therefore must have a sizable force accompany them that can abate such an encounter.
However, their range, which is further than any stationary air defense unit in the game, make them the ultimate siege weapon. It also doesn't hurt that any walls etc do not affect them in any way since they simply fly over them.
This is a video from a guy called "Lowko" that is so unbelievable good at this and I can't believe how quick he is on the keyboard. I, and most of the other people in the world would quickly lose to him.
The point of the story is that if you can develop enough of them and figure out a way to get them across the sky with support (most of which moves faster than it does or will be restricted by terrain) there is very little that your enemy can do to defend themselves against this onslaught.