Summary: I bought this primarily so I could add the new 27" apple cinema display as a 3rd display to my 2008 (3.1) mac pro. My other graphics card is the nvidia GeForce 8800GT which is powering two 19" monitors over dvi. It works fine- no compatibility issues with the nvidia. I'm running Snow Leopard 10.6.5.
Summary: I too own a 2008 Mac Pro and replaced my aging Radeon 2600 with this puppy. First thing I noticed out of the box: it is HUGE compared to ATI and NVidia's older GPUs from a couple years ago. To the tune of 2x as wide and 2.5x as long.
Summary: Before I get into the meat of the matter, There are two reasons why I knocked a star off the rating: 1) Price. Granted, it works with EFI machines, but still it's overpriced. 2) No "official" support for earlier Mac Pros, even though it works fine.
Summary: I purchased a secondhand 2006 Mac Pro with a 30" monitor that originally had only 256mb of vram. Some of the most recent programs such as Autocad 2012 for Mac and Rhino 5 were noticeably laggy when I first ran them, which prompted me to upgrade its video card with the one here.
Summary: I bought this to install as a second card in my computer. the card is fine i just had to do a ton of work to get it to actually work with my set up. The problems i had were more the cause of my motherboard than the graphics cards...
Summary: I bought this as a second card in my desktop. I've had a 5770 for a while and decided to get another for another monitor when I needed it and crossfire for when I didn't need the third monitor. Its been about 4 years since I bought my first and about a year since the second.
Summary: I was going to have to buy a new computer because my old one was "vintage" yeah not cool. but it's working great with this installed! Only issue I had with it was, it didn't fit in all the way no matter what I tried. it still works! but didn't let me lock it in