I have one. Three floors, living + kitchen in middle. Bedrooms top and bottom.
Things I would say :
Negatives.
It's a pain in the ass hauling shopping upstairs, and rubbish back downstairs.
If the bedrooms are on different levels, for example one is on floor 3 and the other on floor 1, some people aren's so happy with having to traverse the stairs to deal with ill kids.
Obviously bringing stuff up the stairs like fridges, sofas to the living room or kitchen can be a bit tricky.
if it catches fire and you are on floor 3 its a long way down.
Positives.
Mine is more secure than a normal house, because it doesn't have many downstairs windows as the majority of the living space is upstairs.
No chance of people like the postman/neighbours walking past and gawping at you through the window. You do feel more secure and removed from the rest of the world living high up in your tower.
Views are better. My house and the houses around it dominate the area. You can't really be overlooked by anyone else - you do the overlooking.
Upstairs living space is pretty warm (downstairs is cold).
You can get pretty fit running from top to bottom.
If you decide to do a loft extension the views would be epic.
Despite the fact people on here don't seem to like them much, there doesn't seem any issues selling them round here and they seem to sell as for as much as the two storey normal houses with the same space.
I would have no issue buying one again.