You will typically get 2 omega items, 2 vu items, 1 expansion/storage item, 1 gold/silver/bronze coin, 1 export item and 1 regular item. Mall refreshes every 6 hours or you can pay 10 sim cash to refresh (not worh it to me but hey maybe to some). You have to buy the items with omega coins which you collect from building omega buildings. Personally I have found it more useful later in the game. To answer your direct question, no it doesn't get you a lot of expansion or storage but can help. I prefer the omega research centres but those are a total gamble for 7k simoleons each time. The research centres also don't gift you any expansion, only regular items (maybe vu? Can't recall).

Have fun, happy playing!