They work well if you take the time to train the dog. If you just throw the collars on and hope it works, it may, or it may not.
You need to walk the dog on a leash up to the flags, and as his collar beeps, pull him back to the house. Keep doing this around the perimeter. Then your train them that the sound comes before the shock, again on a leash. Let them hear the sound, step out and when they receive a shock, immediatly take them back to the front door for a treat. You want them to learn which way to run when they get a correction, otherwise they just run, outside the fence and can't get back in. You have to do this several days then watch and be with them off leash when they get a correction, run back to the front door with them for a treat. The learned responce is to respect the sound, and to condition the habit of which way to run when shocked.
I installed a petsafe fence, it seems to work well. Every couple of years i do seem to have to swap a control module out( they refurbish them).
My brother in law has the Invisible Fence brand , and he has to have them replace the fence controller to every so often.
I think is because we have so many tall trees around us , and lighting runs in on them through the earth, when a close tree is struck.