my friend from Lyon swears by heavy cream and every time I do it, I use heavy cream too. Its not on the light side, yes, but its not supposed to be diet food haha. I also find that lots and lots of fresh ground nutmeg give it that delicious french mountain feel.

I usually use heavy cream and I don't cover. I guess the upshot is that you can do what you like. There's a huge range for success.

You can actually use whole milk and still get nice results, because the starch in the potatoes thickens the liquid -- in fact, I find that cream alone can be a little heavy. I scald the milk, infuse it with a crushed garlic clove, and then pour it over thinly sliced, raw potatoes layered with cheese, salt and pepper -- cover and bake!

Sweet, heavy cream! (At least 42% fat, I'd say...)