The Penn ATPs are harder than Wilson US Opens the first few hours of hitting, but then soften up considerably and are great to play with.
But even in their early stages theyre no where near Dunlop levels of hard.
Right now I'm using a case of Pro Penn Marathons, and although not far off from the ATPs, I think I prefer the ATPs a bit more. Both balls feel the same off the racquet, but the ATPs are a bit firmer and have a better bounce.
The Penn ATPs are the best ball in their price range and one of the best balls overall.