For years I have denied that a potato ricer is necessary for perfect mashed potatoes, after all I like them chunky anyway. But upon testing various potato recipes and personally experiencing the "gummy-ness" that can easier occur when mixing the cooked potatoes, I am now a convert to the need for a fantastic potato ricer. It "rices" the cooked potatoes without breaking it all down so that the final texture when mixed with the scrumptious flavorings (like salt, garlic, pareve sour cream, pareve milk, margarine etc.) is creamy and delicious.
They come in a variety of sizes and price points. The larger ones have the great feature of being able to work with larger quantities but also separate the peel from the flesh of the potatoes so there is no need to peel the potatoes, just boil, rice, and mix in the extras.
I like these two from Williams-Sonoma.com
Potato Ricer  $29.95
Food Mill  $149.95
Although significantly more expensive, this one is from All-Clad and can be used for soups, fruits, and holds a large quantity. Many other companies make food mills for a more competitive pricing like Amazon.com