IMHO the only significant annoyance with DEN is car rental - shuttles often require waiting, long drives and with a rental car i am usually on the wrong side of town no matter where in the area i need to be ... i never had issues with security delays or trains or weather; there are plenty of lounges, a variety of decent food and drinks and the place feels very open airy and pleasant by airport standards. It is definitely one of my favorites among large airports in US.