Here is my Zalman block after ~6 months of use with the Zalman ZM-G200
The loop contained an aluminum Resertor as well as the ZM-WB5 CPU block (alu top) and the ZM-GWB3 GPU block (all alu). The loop would actually have been fine if the base of the CPU block wasn't copper.
As you can see, even after only 6 months there is some corrosion (highlighted in red). Note the red square, that particular corrosion spot almost got to the edge of the o-ring. If it had gone any further it could have potentially caused a leak.
This stuff it supposed to be good for a year, obviously it was not up to the task. Also here is how the block looked like before
So to answer your question - they are not all that great.
Edit: Needless to say, all this stuff is now sitting in a box in an attic. I really wish I had done my research first, would have saved me money in the long run.Edited by GingerJohn - 6/13/11 at 11:28pm