the a/f mixture changes so fast on engines that all these kind of guages do is light up with pretty lights. if you really want to see whats going on, buy on with a lcd display and a separate O2 sensor. that way, you can see the actual number its putting out. like this one:
http://img532.imageshack.us/my.php?image=s256014ii8.flv
its the AEM one on the right.