For me they're one of the best parts about the game. Nothing beats the atmosphere of going on a week long expedition through the tunnels. I just love the lonliness down there.. I feel like a lost soul clinging to the dusty remnants of a long dead civilisation. But the exploration is just awesome too... Bethesda could have easily made the tunnels a straight A-B affair, but we have various exits, junctions, alternate tunnels etc.. the place is like one giant labrynth. And the fact that you cant simply pop into a trader makes it a real fight for survival. There is something eerie about them, walking along a long empty tunnel with nothing but the utter silence around me. That's not even counting the surface level DC ruins with all the random battles you face, Im talking purely the underground parts, the atmosphere is unbeatable.
Just had to share my love of the Metro tunnels....