I invested about $4k into 2 rigs, with 3 7950's each. They've been running for about 1 month, and I've earned 4 bitcoins since. It's a slow payback, but I didn't do this for a fast return. We'll see how well it goes.
Next month about $400, month after that about $300. After that if price does not rise then electricity cost will be close to return and time to power down.