I don't know about the Marhaba one, but I can give you a bit of details for the International Business Class Lounge.

It can get very full around midnight, as passengers from the nearby departing flights to Europe (LX etc.) do get access to it as well.

The buffet is large, food quality seemed quite good to me.

Good wifi, acceptable showers.

Generally, the lounge is rather loud and full, I wouldn't spend 35£ for it for a few hours unless I wanted to eat + drink a lot, plus having a shower..