Went online and ordered a 125-g jar of Marmite from a shipper in England. It arrived yesterday. This morning, I tried it on buttered toast, as suggested by the many youtube reviews of the stuff I'd seen in the interim between order and arrival. This isn't my image, but it's illustrative.

First impression of the smell was familiar to me as a former home beer brewer: spent yeast - but with richer, "darker" overtones. It has a viscosity about like molasses, and color to match. The flavor?
Well, I have to admit that my first taste was not pleasant. The yeast complexity was fine; what almost made me spit it out was the salt. Holy Hannah. I dearly love Asian fish sauce, which I thought was pretty salty stuff. Doesn't hold a candle to Marmite. This stuff is ten times as salty as fish sauce. I don't use much salt on food, so perhaps I'm now a bit over-sensitive to it. But mein gott is this stuff loaded with it!
The jar is on my shelf. I'll probably try it again as a spread - sparingly! But I'm pretty sure that its major use is going to be in soups, stews, chili, and the like. Also sparingly.