Is anyone already using Plack for mission-critical applications and is it something to recommend?

I currently have a Plack app which runs as part of a very, very large image viewing application (lots and lots of scanned images of legal documents). I have a multi-threaded program running with plackup. The web application currently responds to thousands of visitors daily and that number is expected to increase to millions. We have the hardware to throw at it.

That said, if you're doing something "mission critical" perhaps look at launching your *.psgi under Starman instead of plackup. It is "high performance" and has several optimizations.