When I was a kid I used to drive by the Pirelli building all the time and dreading that I might have to go to work in a big, beige building like that one day. I never liked buildings like that until somehow it just clicked for me in my late twenties. Now I think brutalism and similar styles are wonderful.
Anyway, maybe I'm just a weirdo, but it was actually surprisingly pleasant in there: clean and calm with friendly, easy going staff. I kind of regret getting excused a little bit.
Part of what I like about it is that it's not pretending to be something else. It looks like a an extremely secure, resilient building, which is exactly what it is, but it still looks good.
I actually think thats a pretty building, while finding the other buildings mentioned in this thread boring to ugly. Just to put in my two cents. At least this one has a prettier color.