Perhaps more interesting, we use Redis as a caching layer for the entire network. Kyle recently mentioned the specs of the machine in passing. Prior to the NY move our Redis instance was in a virtual machine on a (lightly loaded) web tier machine.
In our (admittedly limited) experience, Redis is so fast that the slowest part of a cache lookup is the time spent reading and writing bytes to the network. This is not surprising, really, if you think about it.