Chase Field (formerly Bank One Ballpark) is the second stadium I have visited with a retractable room. Milwaukee was the first; while I was there the Brewers had the roof opened a sunny 80 degree day. I not a big fan of retractable roof ballparks, but it's necessary in Phoenix and some other cities.
Chase Field had the roof opened when I arrived for a tour at 11:30 in the morning. The temperature in Phoenix that day reached 115 degrees, so a roof is necessary in the Arizona summer.