While these add protection from leaves falling into your gutter the 100% guaranteed is false.
Debris will still accumulate in your gutter with probable blockages and will need to be cleaned . This now will be very costly as the guards will need to be removed to enable access for cleaning .
Another problem to be aware of is the gathering of leaves on top of the guards and backing up on your roof with the potential of causing leaks.
For a small clearing fee yearly your rain gutters can flow free of debris and downspouts flowing effortless.
Go ahead and have a clean when removing debris, this is always recomended for the outside of the gutter, downspouts, fascia and soffit for curb appeal and a small additional fee to the cleaning cost.
Now the last part of the equation...
Cost - The cost to furnish and install the gutter guards does not come cheap in fact quite the opposite.
Rain Gutter clear and clean is still the most cost effective option to maintain your gutter system.