I have a 7-quart slow cooker. I love it. For what they do they are pretty cheap. Only problem with the 7-quart is I have to make a lot of food in it to cook efficiently. It is a really nice problem to have! Almost everything I make can be frozen, thawed and eaten later.
My recommendation is to get a programmable one if you want to cook while you are at work. Preferably one that goes from cooking to a warm setting. The warm setting will keep the food at a safe temperature. The one I have can cook on high for x hours and then switch to low for x hours and then to warm.
I usually prepare the food Sunday night and throw the crock in the fridge and start the cooking on Monday morning. The house smells fantastic when I get home from the gym Monday night.
Don't use a timer switch to turn on your crock pot. You don't want your food sitting around at room temp.
Some of the things I've cooked: pork butt, beef brisket, chicken soup, guinness stew, bone in turkey breast and chili.