I've got some vinyl gutter guards on the front and back of my house, and they've worked really well. The only issue I've had is having these damn squirrels chewing the plastic and nesting inside the gutter. The gutter has a criss-cross pattern opening, covered by a thin plastic screen. The squirrels get into the gutter from the side and then chew the vinyl, and push up through the screen from underneath. We are within city limits, so shooting them is out of the question.
Of course, that's the option I like best. 'Squirrel' is just another name for rodent.
Anyway, if you've had similar problems, what methods have worked? Are there metal gutter guards available? I know they are more expensive, but at the rate that I'm replacing these, it would be worth the cost.