It seems to be working pretty good for me. I'm able to OC my 3870x2 with it. I don't know if I really needed it, probably could have used rivatuner from the start, but it definitely doesn't hurt. One that combined 2 6-pins probably would be best, but I only have two 6-pins connector so I never tried to find one.
You can make one with a little creativity. Land yourself a dual or triple molex to 8 pin adapter, use a molex pin removal tool to move the pins off your 6 pin to the 8 pin, and use the remaining molex adapter to give you the added pins you'll need.