Yes, its stainless steel. I got a free one with a bottle of wild turkey a few years ago. I've been using it for camping and so on. Last October I filled it with Laphroaig when I went camping. I drank most of it, but topped it off when I got home and sat it on the shelf. It just sat there for the better part of 6 months before I decided to see what it tastes like.
At first, I thought the taste had changed due to the little plastic washer insert that seals the lid, but when I compared it to the original bottle, I couldn't tell a difference.
Another reason for the flasks is that when the bottle gets down to the last 1/4, it tends to go back quickly. So, I pour it in the flask so there is little air in contact.
Amazon has a seller ******* them in 4 packs fairly cheap. I got 8 of them and none leak. They're assorted colors of leather wrapping. I put Ardbeg in the black, Caol Ila and Peat Monster in a bronze color, Glenfiddich in green, Glenmorangie in tan, Aberlour in red, etc.