On the question of performance, Riak's FAQ says the following:

We've found it to be fast enough for our purposes, and our goal is not to be "fastest" but rather to stay "fast enough" as the system grows, as hosts fail, and so on.

You know what they say, when you ask about a girl and she's just called "sweet", you know she's not a looker!

Riak's probably not appropriate for anyone without a cluster of over a couple dozen nodes, at least.