I've been using the key in FCP for the longest time. I have Keylight, and it works great as well. The most IMPORTANT aspect of getting a great key, is PROPER LIGHTING. If you're lighting is poor and your screen is not evenly lit, your key will suffer. 4.2.2 footage is going to get you the best results.
Also make sure you have the Color Smoothing plugin first in your chain, then your keyer of choice, and then of course any color correction.