If you use the search feature, you can probably find some of the extensive discussions on bilge pump systems over on our "gear review" forum (unless the threads got accidentally deleted in one of our board crashes).
For your more general questions, do they work, are they worth it: Yes, and yes. Your kayaker buddies will also thank you, because you'll never again have to hold up the group while you stop to dump. The only thing I would say is that if you completely fill your boat up, then the pump will take several minutes to get all that water out. Some folks who are doing hard-core play-boating in their open boat, or running bigger WW, are installing dual pumps to get the water out faster. Just make sure you do your homework, and get batteries that have enough umph.
If you like to tinker, save money by building your own system. If you have more money than free time, then the production units with matched components (e.g., battery sizes, hose sizes, number of fittings required all figured out for you) may be worth a few extra dollars.
I went the "tinker" route - so far, I think my wife's canoe is the only one I've seen with an electronic "auto" switch that turns the pump on when it senses 2" deep water in the canoe, and off once it has pumped it down.
