Cosi (specifically) is a great place to hang out and do some work over lunch or dinner. They have amazing bread thanks to their hearth, which means you can not really go wrong with a sandwich or pizza. If you are not in the mood they have a decently extensive salad bar, or you can have one of their pretty amazing deserts. The way it works for lunch is you go order your food and then pay for it at the register. If you ordered something that needed to be run through the oven then you pick it up after you pay. Once you have your food, find yourself a table and make yourself comfortable. They have sit down service for dinner unless you want to pick up the food. The unlimited refills are always nice, as is the free wi-fi. The staff is friendly, and you do not have to go there to often before they start remembering your usual order. More then that, if a 'new guy/girl' is taking your order one of the people that knows you will give them a tap on the shoulder and fill them in.
My friend Jessica over at Trendy in NYC prefers to go their for dinner because she enjoys the restaurant style service.