I wear a ghillie suit, so I rarely get friendly fired upon (or unfriendly for that matter). It does become problematic when the rules require you to wear a neon flashing sign to identify your team affiliation, however. I have round that the best thing to do is to observe before you fire. In tight situations, when you do not have the luxury, FF can (and will) happen.
I think that marking people clearly cheapens the game. I like the chaos of not knowing. It forces players to think and it forces teams to develop security. How many times has a lone player strolled up to your command bunker and eliminated everyone, despite multiple security check points, just because everyone assumed they were friendly? Those are great moments for everyone.