Slow cookers, were essentially designed to stay on all day while you are at work, so most any model you buy will fill that need. Bert Wolf, author of The New Cooks’ Catalog (Canada, UK) advises you to look for one with a heating element in the sidewall for even heating, both low and high temperature settings, and a removable crock for ease of cleaning. The ones we’re familiar with range in capacity from about 4 quarts to 7 quarts and in price from $25 to $75.
We haven’t done any real research comparing brands, so hesitate to offer much guidance on which brand or model to buy. Some general rules for purchasing mass-market kitchen appliances are probably enough - don’t buy the cheapest one, the difference in quality between the most expensive and the mid-range models is probably not that great, buy the size that meets your needs, etc.
There is nothing nicer than coming home and finding dinner all ready. The caveat to using a crock pot, however, is to use it only for recipes that benefit from long, long, long cooking - soups, baked beans, a cassoulet, etc. Most meats left all day in a slow cooker will be overdone by the time you get home, although Shirley Corriher’s recipe for Fall-Apart-Tender Slow-Roast Pork works wonderfully in a slow cooker, she swears.
In addition to the crock pot itself, you might want to invest in a slow-cooker cookbook. Here are a few choices: