i've not done it, but have heard:
how high up you'll be will really affect what'll grow- going up a few floors is like shifting up a climate zone weather wise, cos the heat from/in the earth isn't keeping your plants feet toasty.
also be aware of stuff tipping over/blowing over if its in pots- gets windy on rooves- picking them up is gonna get really tiresome and disheartening, and i bet the plants won't like doing headfirst concrete dies either, lol.
sounds very exciting though! i bet some alpine herbs would love it up there. you could maybe do a whole green roof dealeo...... make some grass benches to sit on in the summer, etc.