In general, the slower your clock speed, the less power consumption will be used on your device. But if you're lowering your clock speed... then things will run sluggish, especially if you drop below the 500mhz range.
SetCPU has different "profiles" that controls the behavior of your CPU. You can make it run at full maximum performance (for best response times and worst battery usage), but typically you use the default one where it only "speeds up" the processor when it is actually used and needed.
You could, for example, have SetCPU lower your clock speed to something much lower than normal when the screen is off.
Kernels also matter. Some people have made "low voltage" kernels that can help with your battery life. But your mileage will vary with using kernels other than the stock one because they usually go outside the bounds of your CPU's specifications (so a "low voltage" kernel may work well on one device but not another device).
Device manufacturers want you to have a good experience with your device at stock settings. By changing the clock speed (to a lower speed), you should be expecting a degradation in performance at the cost of better battery life.