Interesting. What about Puppet? Also, this really seems like a hammer for small architectures. What is the "critical-mass" where it starts to make sense?
For the critical size in terms of servers, I'd say 1 is enough if you need to start/stop it often. But yeah, the benefits only show up after at least 3 of them I'd say.
I've yet to see a compelling reason to use Chef rather than Puppet, but maybe I've just not been looking hard enough. Puppet is great, and we're a Python shop..