I know everyone uses the term 'fairy tale' to describe Cesky Krumlov - but it really is apt! It's such a beautiful place and looking down from the viewpoints and walking through the streets is so charming. I really enjoyed my visit here.
I only went as a day trip (from nearby Ceske Budejovice, which is a good alternative for accommodation). A day trip was easily enough time to see everything, including the castle. But I do suspect it would be quite nice to spend the night in Cesky Krumlov and explore a bit more when the tour groups all leave. I also saw people kayaking on the river and that would be a lovely way to spend an afternoon, if you had a bit more time.
Other than Prague, this would have to be the most-visited of the World Heritage Sites in the Czech Republic - but for good reason. I would definitely recommend visiting Cesky Krumlov and spending at least the day, if not overnight as well.