I had 3 flights with eznis - from Buyant Uhaa (ULN) to Murun (MXV), from Murun to Dalanzadgad (DLZ) and back to Buyant Uhaa. As a result I had 2 flights delays for 6 hours each (so, the whole day was lost which of course affected my travel plans). Very poor English of the staff even in head-office and nobody knows what is going on. The third trip from Dalanzadgad to Buyant Uhaa wondered even myself. They moved me and my wife from the 9 am flight to the 5 pm flight (most likely to fit into the plane an organised group of tourists) and I found it only when I was trying to check in. I would recommend this airlines only if you are happy to lose 3 full days of your trip and stuck in the middle of nowhere for a day or so. Onboard service however is good enough.
Book well in advance: The cheapest tickets sell first. The policy of lowcost airlines is to sell cheap tickets at first and to gradually increase the price as tickets are bought. Usually, if you buy your ticket 2-3 months in advance, you will probably get the best price.