A good one is very warm. I have a Marmot Zeus, and this is my second Wisconsin winter in it. I absolutely love it. I have a size medium, and it weighs about 15 ounces. Their website says 12 ounces, so mine may need a wash....
It may be a little heavier than some down sweaters you've seen, but considering that Marmot uses 1.1 oz yard (outer) and 1.8 oz yard (inner) material, this jacket has a lot more than 4 oz. of down fill. By my rough guess, I think the medium uses about a yard of each, for a total fabric weight of about 3 ounces. Add zippers and pockets and elastic, you're up to 4-5 ounces tops. That leaves about 7-8 ounces of fill.
(I have no proof of this, just guessing. If someone has hard numbers, please post them!)
The down is top quality, it zips up comfortably snug around your neck, which is important for minimizing heat loss. It also has an elastic hem drawstring, also for minimizing heat loss. This is now an essential feature for me on any coat I want to keep out the elements, or heat in, or both. It helps A LOT!!! It also stuffs into its own pocket.
It blocks wind very well, and with a warm sweater, will do fine with a minus 10 to -15 windchill. But thats me. YMMV. I've got no affiliation with Marmot, just love for this jacket! BTW, I have 4 marmot jackets. Love them all....
And if you get it from REI, you can try it out on a really cold day. If it doesn't keep you warm, you can return it. Just don't wear it half the winter and then return it. IMO, that's abusing their great return policy.