We've owned our REI Sub-Kilo sleeping bags for about six years now, and that means that we've used them on about 750 miles of backpacking trips. Since we usually hike about 7-8 miles a day, that's about 100 nights in the bag, not counting some of our car camping trips.
So as you can imagine, the bags had started to look a little grimy in places. We've meant to wash them for a couple of years, but it's such a major process that we never got around to it.
A visit to REI got us the NikWax soap for down bags, and P filled up the tub and away he went, first washing the bag, then soaking it for a while, washing again, and then seemingly endless cycles of rinse and rinse and rinse and repeat.
Then the delicate process of slowing squeezing most of the water out of the bag, and about 3 hours in the dryer on the delicate cycle.
But it worked. What was a grimy old sleeping bag now looks more or less fresh and new. And we were surprised to see how well they filled out their big "pillow case" storage bags once we had washed them. Before washing, they were not nearly so fluffy--although it's possible we could have fluffed them up a bit in the dryer even without washing them. At any rate, they are now clean!