"I just want to let the devs know that Youporn.com relaunched two weeks ago with Redis as our primary database. With @100 million page views per day, our cluster of Redis slaves are handling over 300k queries per second.

After the switchover we had to add some additional Redis nodes but not because Redis was overworked but because the network cards couldn't keep up with Redis."

So at least the Redis part is I/O bound apparently, that suggests an high degree of efficiency for this use case.