I'm not great at describing such things, but I'll try. It's savory with a touch of nuttiness, very satisfying, complements pretty much anything, and you can mix in things like mushrooms as in the OP and fried onions. It's also pretty healthy compared to a lot of grains. I'd much rather have buckwheat than rice or quinoa as a side dish.

Note that the way my wife cooks it is to rinse and strain it an remove the bad bits, then she cooks it in a frying pan with a bit of water. It's not boiling it like rice, exactly. I'm not sure what you'd call the process. She tries to buy it already toasted (Russian stores sell it that way), but if it's not, then she toasts it in a frying pan for a few minutes. That brings out the flavor.