Adjustable cam gears allow you to tune your cams to your driving style. The difference between 0? and +3? can be as little as 1HP and as much as 30HP, it depends entirely on the build of the engine, the profile of the cams and what you're tuning for. As with most things in life it is a give and take situation though. Tuning for more agressive low end power will usually cost up top.
There's a pretty good writeup on cam tuning here:
http://www.supraforums.com/forum/sho...d.php?t=411951