I agree that Mongolian Airlines are very good. I did the Seoul-UB return trip, 1 way 767 and the other big 787. Economy was excellent, including meals.
2 tips:
1. at UB airport when departing, currency exchange is only available landside, no get rid of excess local money before going through security
2. as mentioned, the traffic nearer to UB is terrible (road is only option). my evening arrival took 1.5hrs to centre (approx 35km - the congestion is say within 5km of city and around); a friend a few weeks later took 3 hrs! In UB the traffic is also terrible despite multi lane roads - walk if you can.
