My original fixie frame was an AL tri frame from the '80s. After that broke (bonded aluminum lugs + 20 years = bad), I got an '02 Marin Verona made of Thron steel. After a taxi killed that, my next (current) frame was (is) an aluminum Basso Coral. Of the three I like the Basso best, and of the two materials my preference runs to aluminum. I'm running 700x23 tires at 120 psi, with aluminum seatpost, stem, bars, etc on roads that tend not to be great. Never have I felt the ride to be harsh. Furthermore, the Basso is noticeably stiffer than the Marin ever was, and I definitely value that. The steel had a nice, distinctive feel to it, but the AL just feels more...I don't know...precise?

I don't know where I'm going with this at this point, so I will just state this: I don't buy the AL=harsh thing. Ride the bike you like and forget the song and dance about which materials are better or worse for whatever constructed reason.