My mom, mid-80s with early dementia, has been living with us for awhile, now, and although it's been a lot of work, things have been wonderful. She was extremely self-absorbed all her life, but now that she has dementia, she has become much more interested in others and has really made an effort to 'roll with the flow' in our household, which is very different from anything she's ever lived in.
One example - we got my mom, the quintessential 'neat freak', a small-breed puppy for Christmas. The puppy was playing with her on her bed and peed on her treasured duvet. I was horrified, expecting a bad reaction, and ran for a towel to clean it up. However, my mom just laughed and said 'he's so cute!' I almost died of shock - this is not the same woman I grew up with.
From my reading I gather that most of the personality changes precipitated by dementia are pretty undesirable, but they don't always have to be. My mom is, for the most part, no longer critical, picky, rigid, anxious, or negative - attributes that characterized her to an almost pathological degree in the past. I have never enjoyed her so much, and we are closer than we've ever been.