I have used it extensively and it works quite well. Its min and max clock values are not specifically 250 and 1000, but instead defined by the slot configuration of your kernel. It's free, simple, and effective. As you note, you can set a different range of clock values for "screen off." This is as close as this program comes to a "profile," meaning it defines speed constraints for a particular condition... in this case, when the screen is off. Thus, you might say it has a "screen-off profile."
SetCPU has some distinct advantages, however. For one, you can configure any number of profiles that will adjust clock ranges based on operating parameters other than screen on/off. For instance, you can adjust the frequency range based on CPU temperature, battery temperature, or battery charge level. If you aggressively overclock, it's nice having SetCPU throttle the processor speed back when it's getting too hot. Also, depending on your particular kernel, SetCPU supports the scaling governors in your kernel, meaning you have control over how aggressively the clock speeds are changed, as a function of CPU load.
So, the overclock widget works great for what it does, but its feature set is limited. Neall