Rating: 5 / 5
Even though the Marmot Grouse is discontinued and only available on Ebay and the like, I just had to review it.
Around 1980 my wife and I were using 3.5 lb REI sewn-through duck-down three-season bags (along with my 6 lb tent, 5.5 lb Gregory bombproof pack, and a host of other heavy stuff).
I called around looking for lighter bags, perhaps less than 2.5 lbs each. Marmot surprised my by suggesting we could reach 2.5 lbs total for the two of us (early ultralight)!
The Marmot Grouse is a semi-rectangular bag with no hood. It can be unzipped and, laying flat, connected to a 9 oz nylon "couplet" that converts one Grouse into a two-person top bag.
Over 25 years later, after frequent use, then little use while the kids grew up, we are using it again, and it still looks like new. And no loss of loft after 25 years.
Our version was "overstuffed" with an extra 3 oz of down (650 fill, pretty good for those days). The purpose was extra warmth and to retain loft despite down deterioration over the years (I see no evidence of this).
We got the GoreTex option, not sure this was the best idea. It improves the temperature rating but increases humidity in the bag noticably. But after my son used it (as a single) on a two-week trip (mid to low-40's at night) we didn't see any loss of loft due to humidity, despite not airing the bag out much, if at all.
If you are in the market for a used two-person bag the Grouse would be a great choice. And it's not too difficult to sew a "couplet" if you can't find one.