IMHO .... if u are first to act, u should fire a bet at the pot and try and take it there.

If u are not the first to act and someone else has bet at the pot, u should only really call if u are getting 4:1 on your money (as this is the odds of u hitting the flush on the next card). Of course, you have a little leeway here because, if you do hit the nut flush, you are going to win a lot more than 4:1 (implied odds)

If u are not the first to act and it is checked to you, I guess you could either bet to try and take the pot down there and then or take the free card, try and hit the flush for free.

Anyway, that's my $0.02 worth...