We have traveled thru Atlanta on every trip to SXM. I have made it on a short connection (90 minutes) before but it was nerve racking for me. We also usually sit toward the front of the plane and are able to exit pretty quickly.

I have seen the custom lines move really well and I have seen them totally stall because of the crowds. We also only travel with carry ons so that is faster. Don't forget if you have any checked luggage you need to go to the baggage claim, pick up all of your luggage, go thru customs, then recheck your baggage for the balance of your flight. I don't think I would push the 90 minute layover if I had to deal with checked baggage. It seems that by the time a person picks up their luggage it puts you quite far back in the line.

We traveled with another couple one year (they had checked bags and we had carry ons only). They ended up getting through the customs line about 20-25 minutes after us. I know this because I was watching the clock to see how much time the checked bag adds. Hope this helps.