Before getting any type of gutter covering, you need to account for physics. If you have a very steep roof, the water is going to flow differently than if you have a shallow roof. I have a steep roof (if it's wet out, the roofing guys don't want to go on my roof). For my house, if there's a solid cover on the gutter, then the water is just going to overshoot and I'll have a waterfall. I did get the gutter covers, but the ones I got are basically chickenwire cut to size. They work great to keep big stuff out. They don't keep little stuff out, so some of that gets stuck and some comes out the downspout.
I had 2 main problems. First, if I got enough stuck on top of the gutter screens, the leaves would block water from getting into the relatively clear gutters and I'll have waterfalls over the edge of the gutters. Quite impressive actually. The other problem is in a few cases, the screens got forced down into the gutter by the weight of the stuff on top (winter, probably ice). Which significantly decreased the water flow inside the gutter until someone got up there to clean them out and fix the screens.
Regardless of the presence of the screens, I find I still have to clean the gutters. How often depends on the trees nearby. When I had 2 mature maples right next to the house, I was cleaning them in spring (seeds) and fall (leaves). When the trees were cut down, I had to clean the gutters (woodshavings). I haven't had enough time since then to figure out the schedule, but there's no longer any significant plant matter getting into the gutters, so probably not frequently.
