We tried the Rainfilter or "GutterStuff" type -
Rain Filter Gutter Filtration for 5 in. K-Style Gutters (8 4-foot Pieces)-5TMG15 - The Home Depot An expanded foam type that goes down in the gutter. Would probably have been ok, except we have 6" gutters, and the ones we bought were designed for 5" gutters. The foam pieces are supposed to lay right at the top of the gutter so that debris can blow off, and the too-small pieces left a space at the top for accumulation.
Wound up replacing them a few months ago with GutterFilters -
Gutter Filter - Minneapolis Gutter Guard, Best Gutter Protection
We had intended to use a similar product available through Costco that seems to be very high quality, but I wanted to avoid installing it myself -
EasyOn Gutter Guard | Home
I believe that the "gutter helmet" type that rely on water surface tension don't work, especially for the short-leaf pine needles and cedar needles we have around here. I believe that roofing gravel off of standard shingles will wash right into them as well.
I had previously used a similarly designed solid PVC aftermarket design like these -
Amerimax Home Products 4 ft. White Solid Gutter Cover-8532 - The Home Depot. The slots started to plug after a few years.
So a screen design was desired, but with a fine enough mesh to exclude the needles and stones. Both the GutterFilter and the EasyOn meet the criteria.
If you are able and willing to install yourself, I think you'd do great with the EasyOn. Give them a call and they will send you a sample piece - very well made and a very reasonable cost. They do have a large size for bigger gutters that is only available directly from them and not available through Costco.
The GutterFilter seems to be good so far - not too much experience yet. I will tell you that having a piece of the EasyOn in your hand when you get your estimate from the GutterFilter sales guy can save you a bunch. Ours came down by about HALF after I told him to forget it, I would do it myself for only $xxx using the EasyOn. Quite a racket!