Do Gutter guards work?

I think I understand the gutter guard concept HOWEVER do they work effectively for collecting and redirecting rain run off from the roof? I've seen the commercials and they look nice and probably work for a nice slow soaking rain. My concern is the heavy rains we can get here in TN. It appears to me that water would merely run off over the top of the gutter guard and over the edge of the gutter. If the 1/2" or 3/4" clearance for water is sufficient with gutter giards, why have a 5" or 6" gutter system on the house originally?

 

In a heavy rain the water will like shoot off the roof that is why we use 5" gutters. 6" for a large roof. The one that works the best is the old open 1/4" mesh over the gutter top for a guard. ED

 

The only gutter guards worth buying and using are the cascading type. The one brand I installed is called "Waterfall".

http://www.water-fall.cc/

The big box stores do not sell these. These work well even with pine needles. I never have to clean out the gutters again.

The only time these have interfered with water getting into the gutter is if there is a valley and you have one of those 1" of rain per 10 minute downpours. There is basically a river of water coming down the valley. I am sure there are some things that could be done to break up the "river", but the once or twice this happened is not worth trying to fix it.