I can't comment on DFW customs and immigration, but every flipping time I fly through that airport on domestic trips, my flight is delayed for some reason. I have reached the point that I just add in an extra 2 hours to my schedule for arrival, assuming that I will get delayed for some reason. And for those bagging on IAH customs and immigration, they are a total cakewalk compared to Miami. I will never ever voluntarily fly international routes coming back through Miami if there is another alternative available. My experiences in Houston have never seemed particlularly long, but I never have flights to catch since I live in Houston, so it might just be that I am not watching the clock as much.
I do know that I can't wait for Southwest to finish the international terminal at Hobby and start flying to Mexico and the Caribbean from there. United will have to step up their game to avoid losing alot of passengers to Southwest once that facility opens.