The best way to complain to United is not to write United at all, but to email a complaint to the DOT complaint hotline and have the DOT forward the complaint to United. The WSJ confirmed this recently in an article noting that the most effective way to get the attention of an airline is to email the Department of Transportation (DOT) consumer division. These are forwarded to the airline for response and are much more likely to get something more than a form response. In the case of United, unless you are GS or 1K, you are virtually assured of a form response if you complain to United through the regular channels. The DOT is virtually your only chance of getting a serious response.
The DOT email is
[email protected]
Good luck!