Having just had my first ride on them today I'm still getting to grips with them (pun intended). I didn't have a pressure gauge with me at work today, so I just pumped them up until they felt firm. Not hard, just firm and that felt good. I could hear the Snap, Crackle and Pop effect, so I knew the studs were doing their thing. Might top them up a wee bit as I hit a bad pothole and I could feel the rim taking a dunt.
I note that you can go all the way to 95psi, but received wisdom seems to be that 'hard' results in the studs not being in contact with the ground.
I defer to the experience of others.