A friend of mine had lots of them in and around his garden (quite a remote place) - the majority appeared at night, as has been pointed out, but sitting outside during beer and BBQ was, for us, uneventful if not a little unnerving. They appeared to be a lot less aggressive than wasps and didn't mind being "encouraged" to move along!
Having said that, i would not like to experience a sting from one! Has anybody been stung by a hornet?!