I bought this block for my Radeon 5870, as I've gotten tired of the noise the stock cooler makes. I know DD makes a block specifically for this card, however I do no believe in all in one coolers. Most of the heat in a video card is produced by the core, and spreading it to the rest of the card (power regs, and RAM) hardly strike me as a good ideal.
With this block on in a loop with a Laing DDC 350, swiftech silent pro 240 radiator, and tdx cpu block, my Radeon's core idles just a hair over the water temperature, and at full load rarely exceeds 40C. The voltage regulators run about the same as with the stock cooler, and the memory and overall PCB is MUCH cooler.
The one downside however, is that this block was frustrating to install. I've often cursed and slandered DangerDen for calling threaded rods a "mounting kit", but in this case, threaded rods that I could pre install into the block would have been a huge improvement over the screws they provided that had to be installed from the underside of the card while holding the block in place.