Sorry for the delayed response, They don't really work at an angle. If you were using it on a counter top, you'd have to point it at the area you would expect the cat to jump up into. To get the most coverage on a counter, it's best to place them facing longitudinally down the counter, about half way between the edge and the wall.
I didn't have any problems with defects, but it sounds to me like people didn't read the instructions and follow them. They're very specific on where to place them, how to turn them on and how long to leave them alone to "calibrate" to their area. If you follow the instructions, they work great.
They can last a long time since it only takes a few times of the cat getting a facefull of air until they quit going where you don't want them. Then you can turn it off and store it and break it out again later if the cat starts going back into that area again. You don't have to leave it in a certain place long term. Good thing too because they're not all that cheap. Well worth the investment though.