Ask the Canadians how well user fees have worked for them. Air Canada blamed the privatization of NavCanada for the majority of it's bankruptcy woes.
As it stands right now, GA is bearing the brunt of fees. Jet fuel purchased by 121 pax operators is not taxed, yet nearly $2 per gallon of jet fuel purchased by GA goes to the feds. In Kansas they decided to give Jet fuel the same fees placed on diesel because the legislature decided that since it could be used in motor vehicles it should be taxed accordingly. Never mind the fact that it's over $3 per gallon more expensive and only available at airports plus they're already charging state and federal taxes that were already in place.
When you privatize a system such as ATC you're opening yourselves up to the lowest bidder. Which will only do just enough to keep things at the status quo or worse yet, cut corners to maximize profits. This is the exact reason that I feel ATC should remain a government function.