In my last post I described the replica Denison Smock I picked up last month, and my trial of Nikwax Cotton Proof water repellency treatment. Last night tested I how windproof the smock is. It was 48*F with a 15 MPH wind gusting to 20 MPH. Wind chill was about 42*. I took a 25 minute walk around the neighborhood wearing only a long sleeve cotton t-shirt underneath and remained comfortable.
Due to the large cut of the smock it'll work well for layering in cool, dry conditions. For example, I have a Columbia Titanium fleece jacket made from Polartec 300, which is nice and warm but not windproof. I also have a lighter weight fleece vest from REI. A combination of one or two of these plus the smock should be warm down into the low 30s, maybe a bit lower. For layering in conditions when it's likely to rain I have a German surplus flecktarn rain parka made of something like Gore-Tex, which will fit over the Denison Smock. (I picked up the German jacket at I. Golderg's last year in virtually-new condition for only $25. I need to go back and get matching pants.)
IMO, the Denison Smock or a similar jacket makes a nice piece of gear for cool, dry conditions. They should offer good wear resistance, and unlike synthetics, don't melt when exposed to flame (though of course it may burn). For military use I'd pair it with wool for layering due to this factor. The Brits still issue smocks, though they've gone to a full zip design and they're now made in the current DPM camo. These are available as surplus for reasonable prices.
For what looks like the Rolls Royce of combat smocks, check out Drop Zone Tactical's Recce Smock.