If I'm not mistaken, this was the first "bong" cooler made.
Basically, water is pumped through the waterblock, then into a spreader of some sort in the top of the bong (Such as a showerhead). There is a pool of water at the bottom, and a fan blowing into an elbow near the bottom. A fan blows into the elbow, which forces air up through the top of the bong. Because the water is split into small droplets by the shower head, it has more surface area, the moving air from the fan takes heat from the water, out the top. The cool water pools at the bottom, and is sucked out by the pump to go through the cycle again.
During this process, water is constantly evaporating.
Ever step out of a pool when it's windy? You freeze! This is the same idea.
The advantages of bongs are that they're cheap, and they can get the temperature of the water below the ambient temperature of the room. Disadvantages: well, they sound like a man peeing. But, this can be taken care of (this is the thread where people told me how to take care of it!). http://forum.oc-forums.com/vb/showthread.php?s=&threadid=71288
Another disadvantage compared to a closed loop system is that water must be added, constantly. This could be a problem if you're forgetful! (no water=fried CPU!!)
In a closed loop system with a radiator, the water will always be above the ambient temperature of the room, unless they're refrigerated or something.
It depends on how much your bong typically evaporates. If it's huge, and evaporates water like there's no tomorrow, then:
A: it's going to be really humid in your room
B: it'll need refilling quite often.
Since I'm going to set up my bong tomorrow (Hopefully ) I'll find this out first hand. rivercom9, if you want me to PM you after a few days of it running, with how much it evaporates, I will.
It would be a good idea to used sitilled water. Using tap water, over time, will corrode your waterblock. If distilled is too expensive, then "spring water," "filtered water" etc etc would probably do.
Don't put water wetter or antifreeze in your bong, unless your want to smell them all day.
Some people put screens over the tops of their bongs to prevent water mist from coming out through the top. If you do that, it'll last a very long time before you'd have to refill it (Perhaps 3 weeks).