I believe it was G. Bruce Boyer who wrote that a grenadine necktie is the best friend that a flannel suit can have. Grenadine is certainly in the top two, along with cashmere. It is the texture, you know.
We have the weather for 14 and 16 ounce (420-480 gram) woolen flannel only a couple months of the year here, but it is unquestionably my favorite suiting. When it is cold, wearing a flannel suit has the positive characteristics of being wrapped in a blanket. Its only negative to my mind is that it takes an extra day of rest after wearing and that can make it less practical than worsteds for airline travel.
Woolen flannel is not the only type of course. The stuff is also woven as a worsted, which makes it amenable to weights under 13 ounces (400 grams). I think of worsted flannels as a compromise though, with all the negative implications of the term. While worsteds do not require the rest of their woolen relations, they also lack texture, not to mention most of the mottling that gives flannel its surface interest. And that same smoother surface means worsteds are also not as warm, which might be OK if one is trying to wear a nine ounce (270 gram) flannel on a sunny day. Personally, in those circumstances I would rather wear gabardine or something else that is meant for warmer temperatures, and save my flannels for the cold.
In the photo, a flannel suit by Thomas Mahon, grenadine necktie from the ASW store and a square from Ralph Lauren.