I tried to get all the diamonds in my second play through. I got all the hidden suitcases but somehow screwed up an assassination mission and didn't get the payment for it. Apparently if you don't kill your target on the first try you don't get paid. Its the only single player achievement I have left.
I got all the diamonds first time round, but not all the tapes and buddies.
I think that with the exception of the Saints Row trilogy, it's my favourite 360 game by a long way. I hope FC3 will be as good.