Decidedly mixed. In the very late stages of Alzheimer's large sections of brain tissue are literally gone. See e.g. here. On the other hand, even with fairly late stage patients they do have better and worse days where they remember more or less, which suggests that some memories are still present. We also know that in some animal models treatment can apparently restore some amount of memory. See for example here (which may be behind a paywall). That last link is to some very new, very recent research suggesting a form of high powered ultrasound may actually help Alzheimer's in mouse models, and there's decent reason to believe that this will work in humans.