I laid some solid oak boards (about 6" wide) using Clip'n'Click from Hoebeek directly onto a concrete floor but I did first lay down a thin sheet of foam because I knew that no matter how flat I thought the floor was, there would always be a clack and a clatter as I walked on the floor and the boards moved downwards just that little bit to hit the concrete.
I also discovered that the concrete floors simply weren't that flat and so, even though the boards were fixed together, they really did float in places with a noticeable downwards movement as I walked on them.
So, this time, I'm definitely going for the self-adhesive foam stuff especially as the area is primarily a corridor and so fairly narrow. If it were a larger room with lots of furniture anchoring the floor down then I'd think again.