They do work but it's much less expensive to buy a (1) man bleeder bottle from Harbor Freight. http://www.harborfreight.com/one-man-brake-bleeder-kit-37201.html
With this device, you simply put about 1/2 inch of brake fluid into the jar, open up the bleeder screw then slowly press your pedal. As the air exits the system, it bubbles through the fluid in the jar and when you release the pedal, the fluid in the jar prevents air from entering the system. I've used thes method for years and the bottle only costs $6.00