I used one of these last time I bled my brakes.
I hooked it up to the caliper and sucked fluid out. I noticed 2 things: somewhere in the line attached to the bleed screw (I think the end piece) was an air leak, which made it hard for me to see if there were air bubbles coming out of the brake lines. Also, the fluid on my car came out on it's own almost as fast as with the pump.
I ended up doing some with the pump and finishing with a gravity bleed. Overall I would say it wasn't really worth it. I ended up giving the pump away to a friend of mine.
I have heard good things about the motive pressure bleeder also.