Opinions on the need for a defragger vary wildly. OS X does auto-defrag small files, and some amount of fragmentation is actually considered good for the system, but that is not to say that the system absolutely can't benefit from a more complete defrag utility on occasion. I've used iDefrag myself and I saw a noticeable improvement the first time I ran it... none since. It's not something you would need to use as frequently as you would in Windows, and how often you would need to use very much depends on how you use your computer. If you are a very casual user, I wouldn't even bother. If you are the type who frequently downloads applications and other files only to turn around and delete them, then you may see a benefit.
BTW... be aware that iDefrag currently has issues with Leopard and they currently don't recommend using iDefrag while logged into a Leopard volume.