Speaking about friendship, Ravi Mohan writes:

One Man Hacking: The Importance of Having Friends Who Disagree

...and it's difficult not to agree.

I'd just add two notes of caution... Remember being not too loud either about

1. Politics, because, in your friends' view, you're as good a person as you're so leftist or rightist as them, or about

2. The entertainment business, aka The Arts (literature, film-making, music, paintwork, etc.) because you'll be as intelligent a person as you conform to their tastes (which, not incidentally, they don't usually regard as tastes, but as statements of culture).

And never, ever, think of giving them your sincere opinion when they've asked you explicitly for it.