To expand on what Abdi said, Plesk 11.5 is fine for production use and has been for some months now.
Indeed, personally I would not use any other version of Plesk when commissioning a new server.
But with something as complex as Plesk, what Abdi is saying is that you might still encounter problems - it is impossible to guarantee a trouble-free upgrade/restore/migration, and as with any control panel there are still going to be bugs. You might never encounter them or they may cause you a problem. But so far I've not seen anything significant. I'm actually quite keen to upgrade my 10.4.4 systems and will probably do so soon.