They're both good and bad. If you have older pipes that are narrower than modern pipes it can take a few minutes for the water to become hot. My parents live in a hundred year old house and installed one, it's nice once it finally warms up, but it can take a good few minutes. During the winter the water is ice cold and takes even longer to warm up.

They are disappointed with it, but they had already sunk like $5,000-$6500 USD on it.