If you have a moderate to large amount of debris, you will not be happy with their performance. As the water nears the opening, it will suck the debris to the devices which becomes matted and clogs them up and restricts the water flow. Instead of a clog in the downspout, its at the opening, that is the only advantage , bottom line, it's still plugged . Your opening to the downspout has everything to do with the performance of your gutters capacity. These products act as a filter and all filters will plug to some degree sooner or later.
Water is a bonding agent.
External gutter leaf guard products come in a wide range of styles and prices at the retail level. Below is a brief summery of those products. Available in rolls of 20'-25', both aluminum and plastic. Hard to install, hard to secure, short life span, but cheap. Available in aluminum, plastic, and steel.