In year round school (some districts call it a balanced school year), the number of days is the same as the traditional calendar, 180 days of school. My niece and nephew have the month of July off every year. They also have breaks throughout the year that are a week or two long. It breaks up the year so that they don't simply go to school for 9 months straight.
My sister loves it! A month off in the summer is more than enough for her kids. It does give them time to travel as a family. The kids can do a week or two of summer camps. One summer she did not sign them up for anything, and they were bored after a few days at home.
At their school during the breaks (but not the summer break) the school offers intersessions. These are optional (and I do believe there is a cost), but very helpful for families where both parents work or single parent families. It gives the kids some down time from their regular classroom experience and time to learn about something not generally taught in school. Sometimes these involve field trips, parents often lead them. My sister was a science major, and she would like to lead an intersession some time.
My sister likes it because the breaks throughout the school year help so that her kids are not so burnt out in May and June. They're ready for a break, but they aren't nearly as mentally exhausted as we were at their age. Also, the summer can be very long, too long for most kids unless you sign them up for a lot of camps. She finds it to be a nice balance.