"For the weight and bulk it isn't very warm. The summer bag will keep you warm down to 50 degrees or so, not much past that. The 3-season bag will keep you warm into the low 30s at best. The -10 rating is for both bags together and the bivy."
This is not my experience with this system at all; I have hundreds of nights of use with it. I can only assume Eric's experience is based on a USGI Military Sleep System that was stored incorrectly prior to sale and that the insulation has degraded to a point it's no longer within specs. But as I'm not Eric, he'll need to explain in better detail what his history is with his gear.
Ft Drum is among the coldest Army stations. I've used this system to, at least, -20*. Important to note, and as Eric mentioned, it is a system so it's assuming that it's being used in conjunction with the bivy and polypro underwear among other things. When on a patrol, I would sleep in it in full BDUs, boots, etc. as a mission requirement.
I currently have the USGI bivy and patrol bag (the green one from the system). I'm fine in the patrol bag without the gtx bivy to 40* in a silk weight base layer. Throw on my down sweater and then I'm good well into the 30s. Remove the zipper and save 7oz. You can use it as a synth quilt and still have the option to use the snaps as a sleeping bag.
As I mentioned above, there's no way to tell how the equipment was used or stored prior to the purchase. You may get lucky and receive one which was never issued and sold through DRMO sales as surplus after the Army made the transition from woodland camo to the digi pattern. You also may end up with one that some chucklehead left completely compressed in the bottom of their wall locker when they weren't in the field. That's the problem with buying surplus gear, there's no way to know how it was treated or used. For example, never buy a tourniquet off of the internet unless you are sure it's brand new. The last thing you need is one that was attached to a GI's RBA exposed to UV light for a tour or two.
There are also some chinese knockoffs out there. For example, if you read through this thread, you'll see that probably half of the people commenting here are not using the genuine MSS:
http://www.ar15.com/archive/topic.html?b=10&f=18&t=664441
How do I know? Because they are unbelievably roomy by design. It has to fit a GI who is sleeping in it with their gear on as is typical when you are on an extended patrol. I'm 6'3", have fairly broad shoulders, and I swim in the one I was issued in the military and the one I bought from Amazon.
For the record, I'm not recommending that you buy this system. At most, I'd buy the GTX bivy and patrol bag to have a 5lb sleep system/shelter combo that should take you down to 30-35* for less than $70 assuming that it's in fair condition.