It seems to vary from person to person how it affects them, but it is pretty much a gradual decline with good days and bad days all along, with eventally the bad days being almost every day, but I know my dad still had an occasional good day even the last month before he passed on. Some people seem to have repeated strokes or seizures, but that doesn't seem to be the case with others.
My aunt seemed to have a much greater degree of memory loss early on, while my grandfather and my dad had more of a tendancy to revisit the past, but still recognize us the rest of the time and be able to carry on a conversation.
My friend's sister who developed it very early in life (before she turned 50) had multiple seizures associated with hers and lost the ability to eat on her own fairly early as well.
The main thing is to love 'em, humor their idiosencricies (sp???), keep them comfortable, and take good care of yourself. When you find that you need help in caring for them, don't hesitate to ask for it. There's plenty available, from many different sources - family, friends, churches, local agencies of various types, etc. If you become a sole caregiver, be sure to get someone to relieve you periodically, even for a few hours daily or weekly, for the sake of your health(and sanity).