Though I am not using one yet, from what I have read, they are very effective due to the length of time the water courses though them giving them plenty of time to remove heat. The form factor is also a good plus. There are a few radiator shoot-out articles around.
Hoot is absolutely correct, the longer the water stays in the radiator, the more chance the water will have to release the heat. The cubes are a bit more difficult to fit into alot of cases though due to the size.
I use the danger den cooling cube and it really works well. I have an InwinQ500 and it fits right above the powersupply, making my water cooled set up completely selfcontained in the case. There was a review on the cooling cube by Joe around the first of the year. http://overclockers.com/articles304/ Nice thing about the cube is that a 120mm fans fits it perfect.
Danger Den for President. They are the best at this point in time. The cooling cube fits perfectly above the power supply as mentioned above. Also using a YS-Tech 120mm fan which also fits perfectly onto the cube. To top this off their water block to CPU clamp is simple excellent. Case is a full tower.
i'm so close to having my water system ready!!! although, i'm becoming kind of partial to the random pile of components just sitting caseless next to my desk. the last few parts should be here any day. i've got my cooling cube mounted and 120mm fans mounted in the case. i used rivots to mount the cube to the case. don't know if anyone has thought of that..but rivots rock! =)
Well, there are alot of factors affecting radiator performance other than the length the water travels throught the radiator. One way the water can be made to stay in the radiator longer is by using larger passages through the radiator. If the passages throught the radiator are larger the water will flow more slowly throught the radiator. This also has the benefit of offering less restriction and therefore resulting in better overall flow and less load on the pump.
Probably the best radiator, in terms of performance, you can possible get for a watercooled PC is a heater core. This is a type of little radiator that is used in cars to provide heat to the passenger compartment. A certain amount of hot water from the engine is directed to the heater core and a fan blows air through it into the passenger compartment of the car. Anyway, heater cores make great radiators for watercooled PCs. They can also be obtained rather cheaply. I bought a heater core for a 1980 ford mustang from Autozone for less than $20. You may also be able to find used ones at an even cheaper price, but I'd just get a new one. Many of them are made of copper and therefore should be used with a copper waterblock. Although I have seen Aluminum ones as well. You might also be able to find one on ebay at a decent price. The only downside is their size and the fact that they may not be as visually appealing. Some of them might be just a wee bit larger than some of these other radiators that are often used for watercooling and therefore may have issues fitting into a computer case.
Anyway, whatever radiator you use one thing to keep in mind is that if you use a copper waterblock you should use a copper radiator and if you use an aluminum waterblock you should use an aluminum radiator. If you mix copper and aluminum together in your watercooling system you will get corrosion and the aluminum will be slowly eaten away due to an electrochemical reaction.