My take is that I never just eat lettuce on it's own, at all, and so as they are not the tastiest of things..... I would say that the texture is the important thing for me, and biensur a good vinaigrette.
I do put it in sandwiches which always have sauces in them too.
So, I have not noticed any difference. And the frisee, which is bitter is just as bitter here too, and I always mix it in with other leaves, as I don't like it on it's own.
When we can, we buy endives here and they are just the same, poached or in salads.