I work downtown, and this is one of our regular haunts on Fridays when we go out as a group.
I typically get the Skyscraper on an onion roll. It's piles of corned beef and pastrami, topped with swiss, on an onion roll, then served with french fries and a pickle spear. The guys and ladies behind the counter are always awesome, and they even come around and pick up your trays when you are finished!
Though it's not the cheapest lunch (around $10-11 for my lunch with a drink), it's not out of the normal range for lunch downtown. And you will never go away hungry. This is definitely one of those places you will go and need to take a nap after lunch.
I can't recommend Downtown Deli enough, it's one of my favorite places to eat in Downtown Phoenix.