This is by far the hardest FPS I've played till now. But it still rocks!
The battles are more intense from the original and the difficulty soared from what you think you have been through from those FPS experiences of that time. Ok, the original Doom is mad, but this one can also make you grit your teeth if you're the impatient sort. As in the first Medal of Honor, you can't save your progress during the missions and this time round, most of the missions at the later stages are longer.
Not to be a spoiler, it's safe to say that there's a weird twist to the later missions that appears sort of whimsical and far-fetched. As in historic shooters like later MOH series and the Call of Duty series, they put you in the World War II theatre in realistic surround sound and traumatising visual experiences, allowing you to witness the chaos of WWII in a form as true as can be, but these sort of bonus missions in Underground puts you out of the realism in a snap. You'll know what I mean when you have a chance to get there and find out for yourself. And these missions are probably one of the hardest scenarios you can find in your FPS gaming history I promise. So much so that you will buy yourself a real medal on eBay to commemorate the day you conquered the game.
KingLim