aldive, one thing to remember is if your vehicle is still under warranty (not sure if it is, though its a 99 so its probably expired) Ford strongle urges people not to use these. Same thing with throttle body cleaners. Apparently they come from the factory coated with some sort of a protectant, and by using them you take away that protective layer. I don't care though. On my explorer I use throttle body cleaner all the time because it gets so dang dirty in there...
as for fuel injection cleaners... I've used them before but never noticed much of a difference. But if they start getting clogged you could run into loss of power, engine wear, etc.