I always thought the 6xxx series are much worse than the 5xxx series because of less shaders and crappy VLIW4 not VLIW5 like the 5xxx with SDK 2.1 and downclocked memory.
Bullshit spread by individuals who forget to shut up when they have no idea what they're talking about.
A hd6770 (juniper-xt) is basically nothing more than a re-branded 5770 with some minor bios tweaks completely orthogonal to mining. So are 68xx cards (barts).
VLIW4 starts with cayman (hd6950).
Yes, 6870's are actually quite efficient miners and are often overlooked. They are cheap, low-power, and overclock well. It is not a problem getting 300Mhash/s out of a 6870, and it won't fry eggs while doing it.
6950s and 6970s are faster overall (350Mhash/s for a 6950), but they are quite a bit more expensive and use a lot more power to get those rates. They are not as efficient from a $/Mhash or $/W perspective compared to a 6870.