I used the marhaba services last October. I had arrived at DXB from LGW around 8pm which it was crazy busy. The plane had landed next to a gate where literally the queue is for Fast track immigration. Anyway I was met by the representative which was a very lovely lady from south east Asia. She took us to a Immigration line with no queue and then headed us to baggage where they have someone to collect our bags, and then onto EK chauffeur arrivals lounge. She stayed with us all the way till it was our turn for a vehicle which was nearly 30 mins!
The only thing worth it to me as much as the lady was lovely and helpful was the immigration. Even EK fast track had a long queue of people waiting but obviously I dont know how long it took for those to get through if staff are efficient. The baggage and walk through was just a bonus but its really not needed if you fly J. If you are like me, you dont like queues, you get annoyed quickly and you just like to be efficient in time and not bothered about an added cost I think you will find its worth it for arrivals at a busy time in DXB.