I am probably old-fashioned, but I do prefer the union suit to more modern thermals. This one fits well, keeps its size well after a wash, and is much more comfortable (to me) than the waffle-weave on most other thermals.  It won't do well for long-term exposure to sub-zero weather but for most other uses or even slightly more moderate temps it is a good suit.  Fair warning though: it is not a good match for women (per my wife). The access points do not line up well for women to easily use the loo.