As far as opensmtpd is concerned, it is production ready and has been used in high volume environments sending mails to millions of recipients daily, handling multi-million queues.
The project was started in 2008 so when it comes to being run in production environments, I think we can claim that it's ready :-)
Now as for the what you would gain, I won't do proselytism, we have published a couple presentations on our website which describe how it works and some of the technical details behind it. You will also find lots of technical details on my blog where I post mostly about OpenSMTPD internals.
If you have a running Postfix, I suggest you don't shift but keep your working setup. If you intend to setup a new MX, you should definitely give a try and see how it goes for you.
Could someone explain why one might want to use qmail (or this, its newest incarnation) over postfix or opensmtpd? Would I gain anything by shifting to qmail or opensmtpd?