The stock chroma key effects in AE 6.5 or 7 are adequate tools, and if you have the patience and know-how, you can make them work. I've had to use them in the past, and while they're a little more finicky and less refined than some of the other options mentioned here, they can get the job done.

I will also add that one of the cool things about pulling a chroma key in After Effects is that you can apply the chroma key effect to a layer multiple times. This is useful if the photographer didn't have a blue/green screen lit properly or if the first effect isn't keying out enough of what you need. In some cases, I would duplicate layers and create mattes in tandem with the chroma key effect to pull cleaner keys.

Re: Blue screen Keying Software for Windows / Adobe PreimierCS3 ?
 

I use keylight in AE. It works great.

Here is a tutorial a showing how to use it: Keylight Keying Tutorial .