Here is what I use and I get excellent keys. I usually use the DV codec. Apply filters in this order: Color Smoothing 4:1:1 > Color Keyer (adjust edge feather to zero to start) > Matte Choke > Matte Feather. Adjust the Color Keyer to your key color, tweak and then add the choke and feather to tweak the edges. Don’t forget that you can add color correction at any point. Also, adding color correction to the background can help. Sometimes you can get a good edge but something drops out in the middle. Just duplicate the layer with no keyer and use a garbage matte to trim to fit. There are free garbage matte filters online that have more than 8 control points. Search for “free FCP filters”. I have gotten clean keys with this method with green, blue and white backgrounds. You need to experiment. I think many people give up too soon. Give it a try.