I have made this soup numerous times now as we make it regularly at our local TMX Back to Basics cooking class.
I love the taste of the soup, although I can't be sure that I have ever found exactly "butter lettuce" variety. It is always just some sort of lettuce and hoping for the best, but it has been delish anyway.
The soup doesn't make a lot, being based on only 500g water, so it would be very handy one for single people using the Thermomix.