Had a little time on my hands, so I tested a few apps (xBack, Visage, DeskSaver, DesktopSaver, and Wallsaver). xBack and Visage are shareware, the others are freeware. In a nutshell, you get what you pay for. xBack and Visage are feature rich compared to the others. I wanted to like xBack, but with activity monitor opened to watch the amount of real memory usage and CPU, I was quite surprised. With xBack running, switch around between various screen savers. Doing this, the real memory jumped from around 20mb to more than 300mb, and as somone else stated, CPU usage was around 40% or more. Also, after switching screen savers a few times, some of them failed to load the second time around. It was enough to give me cause for concern. I am running an intel iMac with 10.5.2. Visage had none of these problems, plus it offers more features to customize your mac. It is a preference pain, not a stand alone app. One of its preferences allows you to choose 5 different levels of CPU usage. I used the lowest setting, switched around my various screen savers numerous times, and the real memory stayed around 20-30mb, with CPU usage dependent on the specific screen saver. A more complex saver used more CPU, but when you chose a simple one, the CPU usage came back down.
As for the freeware, they all worked to get your screen saver on your desktop, but that was basically it. No ability to change savers from the program, can't look at the individual savers options, no randomizing, etc. But they worked, and their screensaver engines used roughly the same amount of memory/CPU as Visage. My favorite of the free group was Wallsaver.
But if I get such a product, Visage would be my #1 choice.