I've been using the NXTCam for a week or so now. It does pretty well what it says on the box but with a few caveats. The color discrimination performance is poor. The camera is very sensitive to red, and barely sensitive to blue. I've tried it in a few different lighting conditions. It is better under flourescent light, but still not good. I've been using a black ball against a pale background which works well. Also the blobs can report erroneous boundaries near the edges of the image. The documentation is poor (and wrong in places) and you need to be prepared to work things out for yourself. But if you're using Robot C, your probably used to this ...
Still my kids were very impressed to watch the robot chase the ball across the carpet! Speed of response is certainly not an issue. All in all, good value at the price, but I hope they straighten out the bugs and documentation soon.
I've written my own library which sets up the camera and grabs the blobs into an array. As other posts have noted, you will need to use nxtcamview to set up the color spaces (although you could probably write a Robot C program to do it as well). I also have some demos that display the blobs as text, display the blobs graphically and chase a blob around. I'll post these on my website soon (stuck behind a foreign firewall right now) and send a post to the forum with the URL when done.